# Cheapest way to get overdrive



## xconcepts (Jan 4, 2009)

Guess my tires are smaller than I thought cause at 70 I'm near 3800rpms. So I'm actually thinking about trying to get overdrive. Not sure where to start though, 200r4, T34....etc. Whats the easiest and cheaper way to get overdrive?

X


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## Rukee (Feb 8, 2007)

Really, REALLY tall 15" tires. 



Cheapest way I suppose would be to just swap out the rear gears for more highway friendly ones.


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## xconcepts (Jan 4, 2009)

Yeah, I just went from 2.56 to 3.55. I have a set of 3.23's that don't fit either carrier I have even though they were pulled from a 66. I guess his was an early 66 and mine a late 66:confused Will be in the classifieds after I get a pic.

I was considering a bigger tire, but want to keep that off the line speed of the 3.55(hell of a difference btw). Just trying to get options for an overdrive.


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## Rukee (Feb 8, 2007)

I have no problem smokin the tires with 3:55s in first and second, heck, sometimes I even get a third gear scratch. That's with big fat 15's.


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## busaben (Oct 30, 2008)

im not sure if it helps any, but i got a slightly used gear vendor unit behind my 350 turbo. i didnt want to loose my dual gate shifter and wanted overdrive, have yet to install it though. price was $1500 i believe, thats about the same as buying a built 4 speed auto. im also running 3.55's


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## 68greengoat (Sep 15, 2005)

Just out of curiousity, what size tires are you running, X? I believe somebody posted a link to site that calulated RPM's given the tire size and gear ratio? I can't seem to find it tho....


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## Eric Animal (Oct 28, 2007)

Anyway, the cheapest way is probably a 200r4 swap. not necessarily the BEST way, but economical and not too difficult. E


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## Eric Animal (Oct 28, 2007)

busaben said:


> im not sure if it helps any, but i got a slightly used gear vendor unit behind my 350 turbo. i didnt want to loose my dual gate shifter and wanted overdrive, have yet to install it though. price was $1500 i believe, thats about the same as buying a built 4 speed auto. im also running 3.55's


 SHIFTWORKS in New Yawk, makes a kit that puts another detent in a Dual Gate shifter. This allows use with a 4 speed automatic!


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## 68greengoat (Sep 15, 2005)

Here's one of the links to a calculator. You can enter desired RPM, MPH, then different tire sizes to get the 3.55 ratio. Ex: by using a 26" tire at 70MPH w/3.55 gears you would be at approx. 3200rpms. Changing the tire size may be the cheapest route.
Differential Gear Ratio Calculator


It was in one of your earlier threads:
http://www.gtoforum.com/f12/rear-end-question-23269/


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## ALKYGTO (Mar 29, 2010)

OP, looks like your running a 22" tall tire :confused? Or you have a really loose converter. I don't think you need an OD, you need to find out what is slipping in the tranny you've got.


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## xconcepts (Jan 4, 2009)

I haven't measured my tires, they probably are 225/55/15 or so. I'm going to measure them tonight and probably get another set to get up to 27". I've got a 4 speed manual not auto.


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## geeteeohguy (Feb 2, 2008)

Cheapest way, with your gears? Run a 27 inch tall tire. That will bring you back down to around 3200 rpm at 70mph. Stick car? Run the 3.23 gears. 3.23's will require a 3-series carrier. Right now, you are running a 4-series, and you removed a 2-series (2.56). I'm running a 3.36 in my '65 with 4 speed and 27" tires in the back....3000 rpm at 73-74 mph. My 2.56 equipped, automatic '67 with 26 inch tires is cruising 92mph at 3000 rpm.


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## mighty454 (Mar 31, 2010)

Eric Animal said:


> Anyway, the cheapest way is probably a 200r4 swap. not necessarily the BEST way, but economical and not too difficult. E


Is it not better to go to a 700-R4 and maybe cheaper also? All the 700-r4 I've found are cheaper that the 200-r4's. I also read somewhere that the 700-r4 can handle more power but you have to make sure the throttle cable is adjusted perfectly?

I'm looking to get rid of my th-350 and put an adapter plate in a go with my best choice of an overdrive for my pro-tourer.


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## xconcepts (Jan 4, 2009)

geeteeohguy said:


> Cheapest way, with your gears? Run a 27 inch tall tire. That will bring you back down to around 3200 rpm at 70mph. Stick car? Run the 3.23 gears. 3.23's will require a 3-series carrier. Right now, you are running a 4-series, and you removed a 2-series (2.56). I'm running a 3.36 in my '65 with 4 speed and 27" tires in the back....3000 rpm at 73-74 mph. My 2.56 equipped, automatic '67 with 26 inch tires is cruising 92mph at 3000 rpm.


I figured the 3.23 gears I bought came with the carrier, which is now in the classifieds, would work. Thats why I bought them, but they didn't fit so I went with the 3.55. I'll probably just get the bigger tires.


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## Rukee (Feb 8, 2007)

Big Tall FAT tires!!! :cheers


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## Eric Animal (Oct 28, 2007)

mighty454 said:


> Is it not better to go to a 700-R4 and maybe cheaper also? All the 700-r4 I've found are cheaper that the 200-r4's. I also read somewhere that the 700-r4 can handle more power but you have to make sure the throttle cable is adjusted perfectly?
> 
> I'm looking to get rid of my th-350 and put an adapter plate in a go with my best choice of an overdrive for my pro-tourer.


You can get a 200r4 to handle a lot of power, also the 700r4 has a Chevy bolt pattern so you need an adaptor plate. the tranny cable MUST be properly set or you will cook the transmission.


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## Eric Animal (Oct 28, 2007)

xconcepts said:


> I haven't measured my tires, they probably are 225/55/15 or so. I'm going to measure them tonight and probably get another set to get up to 27". I've got a 4 speed manual not auto.


Then you should call my friend Paul at Medatronics Corporation's 5 Speed Home page! he has been building standard trannys for a loooooong time, he even has a published book on the subject. Eric


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## xconcepts (Jan 4, 2009)

So measured my tires......26"....so why am I running so high on RPMs?


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## 68greengoat (Sep 15, 2005)

That does sound kind of screwy. If you punch in your numbers in the calculator, 70mph, 3800rpm and 26" tire they say you should have a 4.20 gear in it. Maybe the calc is off..... You didn't slip in 4:10's by mistake??? I know you didn't, but....... Tach is off? But still, it should sound like the motor is winding up pretty tight....


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## xconcepts (Jan 4, 2009)

The guy I bought it from is 100% its the original rear end and my PHS states 3.55


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## Rukee (Feb 8, 2007)

If it's an aftermarket tach is it set to 4 or 6cyl?


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## xconcepts (Jan 4, 2009)

aftermarket, set to 6 as far as I know. Didnt have and RPM issues when I had the 2.56. Running at 70 I was at 2400-2500 or so.


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## 68greengoat (Sep 15, 2005)

xconcepts said:


> The guy I bought it from is 100% its the original rear end and my PHS states 3.55


You could always spin the tire and count the driveshaft revolutions to double check..... May be worthwhile at this point.....


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## jetstang (Nov 5, 2008)

Eric Animal said:


> You can get a 200r4 to handle a lot of power, also the 700r4 has a Chevy bolt pattern so you need an adaptor plate. the tranny cable MUST be properly set or you will cook the transmission.


But, the tranny adapter is under $100 to adapt the 700R4 onto the pontiac, so not a show stopper. I don't know about torque load, 700 should be healthy enough for a cruiser.


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## Rukee (Feb 8, 2007)

xconcepts said:


> aftermarket, set to 6 as far as I know. Didnt have and RPM issues when I had the 2.56. Running at 70 I was at 2400-2500 or so.


If it's set to a 6cyl, it won't read the right RPMs for an 8cyl. Just saying....


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## xconcepts (Jan 4, 2009)

sry ment it was set correctly for the 8cyl when the 2.56 rear end was on.


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## ALKYGTO (Mar 29, 2010)

So I see you say you have a manual tranny. Is there clutch slippage? 

As far as overdrive manual trannies, I would recommend the Tremec TKO. Basically a bolt in (need to shorten the driveshaft and change the front yoke) and it's a sweet shifting box that will take lots of abuse. Not cheap, but worth every penny IMO.


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## Rukee (Feb 8, 2007)

Well, I have 3:55s and 295/50/15s and run under 3k @ around 75mph.
I'd jack up the rear and spin the tires and verify the ratio.


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## mighty454 (Mar 31, 2010)

jetstang said:


> But, the tranny adapter is under $100 to adapt the 700R4 onto the pontiac, so not a show stopper. I don't know about torque load, 700 should be healthy enough for a cruiser.


So If I do the swap for a 700-r4 what's the best gear to go with you think to get the best of both worlds as far as off the line torque and decent rpm's at high speed?

I'm currently upgrading my to a 3 series 8.2 posi unit but don't know what gear to select yet?


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## geeteeohguy (Feb 2, 2008)

Mighty, if you're upgrading your 3-series and are going to run an overdrive, put the shortest gear you can in it: a 3.23. If I were going to run an overdrive, I would be running 3.55 or 3.73 gears, though. That would take a different carrier. X-concepts, are you sure your tach is accurate? My '67 with 2.56 gears is at 2450 rpm at 75mph with 26 inch tires....your rpm sounds a little high...not much, but a little.


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## mighty454 (Mar 31, 2010)

geeteeohguy said:


> Mighty, if you're upgrading your 3-series and are going to run an overdrive, put the shortest gear you can in it: a 3.23. If I were going to run an overdrive, I would be running 3.55 or 3.73 gears, though. That would take a different carrier. X-concepts, are you sure your tach is accurate? My '67 with 2.56 gears is at 2450 rpm at 75mph with 26 inch tires....your rpm sounds a little high...not much, but a little.


My understanding on the gears is that a 331 and lower (3.55 3.70 3.90) will work with the 3 series carrier. Is my information off because I certainly want to stay away from buying the incorrect parts.

The splits on the gears as I've researched are as follows:

series 1 2.56 - 2.73

series 2 2.93 - 3.23

series 3 3.31 and up

Any advice or corrections are always appreciated.


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## geeteeohguy (Feb 2, 2008)

4 series is 3.36 and up, from what I've learned. The 3.31 was a Chevy ratio often found in 12 bolt rears.


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## geeteeohguy (Feb 2, 2008)

There is no series 1. Series 2 is 2.56-2.78; series 3 is 2.93--3.23, and series 4 is 3.36--4.33.


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## mighty454 (Mar 31, 2010)

geeteeohguy said:


> There is no series 1. Series 2 is 2.56-2.78; series 3 is 2.93--3.23, and series 4 is 3.36--4.33.


Okay, I just emailed the guy that I'm buying the posi from. He said the gear he used to have on it was a part number - 37 11 68 gm - He said it was it was a 3.36 but he may be confused on the series because he figured it was a 3 series. 

I guess I'll have to try and find what that part number goes back to.


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## geeteeohguy (Feb 2, 2008)

Mighty, 37 11 is indeed a 3.36. So, with that carrier you can run 3.36, 3.55, 3.90, 4.11, or 4.33 gears! A 3.36 is an excellent all around gear, good with sticks and automatics. If you're going to run an overdrive, consider a 3.55 or maybe even a 3.90, depending on the amount of OD. Good luck.


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## mighty454 (Mar 31, 2010)

Thanks for all of the info. It's looking like I may go for the 3.55 so I can stay highway gas milage friendly but still get some decent torque down low.


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## Rukee (Feb 8, 2007)

Rukee said:


> Well, I have 3:55s and 295/50/15s and run under 3k @ around 75mph.
> I'd jack up the rear and spin the tires and verify the ratio.


Correction, I'm just a tick over 3k @ 75 MPH.


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## xconcepts (Jan 4, 2009)

well @ 60 I'm @ ~3K. Calculating that I'm running 3.88's??? Gonna lift it today and rotate the tires. 

I've been having issues with my 2nd and 3rd gears sometimes. When I stomp on it 2nd and 3rd hesitate where it seems I'm not getting fuel. I have to back off the gas completely and slowly press it back down. I know it need rebuilt and was told that the floats need reset and its not getting the fuel it needs. Thats why I'm getting that hesitation and that is affecting my RPMs.


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## Rukee (Feb 8, 2007)

xconcepts said:


> well @ 60 I'm @ ~3K. Calculating that I'm running 3.88's??? Gonna lift it today and rotate the tires.
> 
> I've been having issues with my 2nd and 3rd gears sometimes. When I stomp on it 2nd and 3rd hesitate where it seems I'm not getting fuel. I have to back off the gas completely and slowly press it back down. I know it need rebuilt and was told that the floats need reset and its not getting the fuel it needs. Thats why I'm getting that hesitation and that is affecting my RPMs.


Sounds like the accelerator pump is not working. Remove the air cleaner open the choke and verify that there is fuel squirting in when you ratchet the throttle open.


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## geeteeohguy (Feb 2, 2008)

I agree. Sounds like the carb(s) aren't getting enough fuel or are starving for fuel. Float settings and accellerator pump operation are critical.


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## 68greengoat (Sep 15, 2005)

xconcepts said:


> well @ 60 I'm @ ~3K. Calculating that I'm running 3.88's??? Gonna lift it today and rotate the tires.


Let us know what you find. I know some of us are curious........


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## xconcepts (Jan 4, 2009)

Here now, it turns ~2.75 per rotation.


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## xconcepts (Jan 4, 2009)

ok, I think I know what is going on. Last june was when I bought the car, I paced someone and found out that it was about 15mph off. The speedo went out and just got fixed when I had the rearend done. I guess I though it would fix the difference in it being off, I am probably wrong though. I need to pace someone this weekend and see if I'm still ~10-15 off. If I am than I'm good, but I still don't understand the rotations then. I verified with the guy that replaced my rearend and he counted the teeth and came up with ~3.54 so I know its a 3.55.


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## jetstang (Nov 5, 2008)

mighty454 said:


> So If I do the swap for a 700-r4 what's the best gear to go with you think to get the best of both worlds as far as off the line torque and decent rpm's at high speed?
> I'm currently upgrading my to a 3 series 8.2 posi unit but don't know what gear to select yet?


I have a gear vendors OD in my 454 SS w/3.73 gears and it's about 3000 to 3200 at 70, speedo is at 85. So, w/3.73's you still aren't highway cruising like the new cars that run 2400 at 80 and get 25 MPG, but it's way better than 4000 RPMs at 70. But my Vette has 2.76 or 3.08s w/OD so, 2400 is about right I guess. I thought it was 3.08s, but sticker in glove box got rubbed where the gear code is...


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## Eric Animal (Oct 28, 2007)

3.73 x .75(OD)=2.79:1 final drive........big difference. This is with out tire height figured in......


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## Rukee (Feb 8, 2007)

xconcepts said:


> Here now, it turns ~2.75 per rotation.


Something's not right. If you had 3.55s then the drive shaft should turn just over 3 and a half times for every one revolution of the tires, not 2.75 turns.
Even at 2.75 turns your RMPs should be lower then they would be @ 3.55s.


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## geeteeohguy (Feb 2, 2008)

Two things need to happen here: the rear cover needs to come off and the numbers on the gears need to re read, (or teeth counted) , and a new, correct speedo gear needs to be installed and verified. In CA, we have all those electronic "YOUR SPEED IS" signs everywhere, which measure your actual speed with a radar or sensor. I use those to verify my accuracy. The factory manual will tell you which speedo gear you need with which rear gear. No reason to drive around with a speedo that's 10-15 mph off. That's just bogus.


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## jetstang (Nov 5, 2008)

geeteeohguy said:


> No reason to drive around with a speedo that's 10-15 mph off. That's just bogus.


OK, my 90 454 SS has an electronic pickup for the speedo, and the car has factory 3.73 gears, so it should be right. I contacted gear vendor and asked them how to fix it and they didn't know. Any help is appreciated.
Actually, I use my GPS for my speedo now, works great.
I do need to know which color I need for my 70, it had 2.76s, now 3.36s w/ 275 60 15 tires.


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## 68greengoat (Sep 15, 2005)

geeteeohguy said:


> No reason to drive around with a speedo that's 10-15 mph off. That's just bogus.


Oh, it's not that bad as long as you know how far off you are. Mine has been off 8 for years. I usually don't see freeway driving, so I just put it on 50 and cruz.....


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## xconcepts (Jan 4, 2009)

Teeth were counted when they were installed, guy guarrenteed they were 3.55's. I forgot about the one on my gps, i'll have to drive it around and see.


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## jetstang (Nov 5, 2008)

68greengoat said:


> Oh, it's not that bad as long as you know how far off you are. Mine has been off 8 for years. I usually don't see freeway driving, so I just put it on 50 and cruz.....


Greengoats got a granny gear!! lol
Makes for some great top end. What did you take out/put in? Bet that thing will smoke the hell out of the tires with a 462!! :cheers


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## 68greengoat (Sep 15, 2005)

It came with a 2.78 and it's still in there. 3.55's are going in as soon as I have some extra cash to burn......


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## jetstang (Nov 5, 2008)

68greengoat said:


> It came with a 2.78 and it's still in there. 3.55's are going in as soon as I have some extra cash to burn......


So where do you drive 50 mph at? Hopefully not on the interstate, do people wave at you like you are in a model A? Oh look at that cute old car with the 462 CI motor, lol.. 2.78 should cruise nice on the highway.


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## 68greengoat (Sep 15, 2005)

When I set it on 50 it's actually 58. Going slower gives them a better look. They'll even ride next to me to make sure I see their "thumbs up"..... No freeways around here. If I do find one, I boost it up to around 60 which is actually 68. That 2.78 does go down the road nice. The R's are pretty low...............


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## Rukee (Feb 8, 2007)

I've seen some little gear reduction blocks that screw onto the tranny then the speedo cable screws onto to that used on `70-`80s GM 4x4s that will speed-up or slow-down a speedo. Might wanna check at some local salvage yards.


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## geeteeohguy (Feb 2, 2008)

Jetstang, what trans are you running? I can get access to a '67 factory book I have at work that has all the gear charts. I'll have to dig a little.....it'll be tomorrow or the next day. It was how I found the exact gear and transducer for my '67 when I swapped from a 3.36 to a 2.56. I got the gear online for like $6, and the transducer for $12. You will need a transducer, at least if it's an auto, as GM used two, one for the tall geared cars and one for the short geared cars. I know you have a '70, but the '67 manual should work, as the "math" is the same! I think my 3.36 speedo driven gear was 43 teeth and brown in color, and the one for my 2.56 is 36 teeth and natural in color. My speedo is now dead-nuts accurate. I'll get back to you and let you know what I find.....


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## jetstang (Nov 5, 2008)

geeteeohguy said:


> Jetstang, what trans are you running? I can get access to a '67 factory book I have at work that has all the gear charts. I'll have to dig a little.....it'll be tomorrow or the next day. It was how I found the exact gear and transducer for my '67 when I swapped from a 3.36 to a 2.56. I got the gear online for like $6, and the transducer for $12. You will need a transducer, at least if it's an auto, as GM used two, one for the tall geared cars and one for the short geared cars. I know you have a '70, but the '67 manual should work, as the "math" is the same! I think my 3.36 speedo driven gear was 43 teeth and brown in color, and the one for my 2.56 is 36 teeth and natural in color. My speedo is now dead-nuts accurate. I'll get back to you and let you know what I find.....


I appreciate the help. It has a turbo 350 trans.


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## geeteeohguy (Feb 2, 2008)

Jetstang, I found a 1970 chart online at www.murraysimon.com/GR04_ILL.pdf
I did some math, and your 275/60/15's are 28 inches tall, 2 inches taller than stock. This makes your 3.36 rear gear a 3.10 ratio "on the road". You would be at 2600 rpm at 60mph with the stock 26" tire, but with your 28's, your at 65 mph at 2600rpm. That said, It seems that a speedo gear for a 3.08 rear end application would be the closest you could do running your big rear tires. The chart says it's a 43 tooth purple gear. The one in there now, for your old 2.78 gear, should be a 38 tooth blue gear. Check out the info online, check out TCI and other sites, and you should be able to get it close for very little cash. This sounds about right, as I did a reverse swap, pulling a 43 tooth gear from my 3.36 convertible and replacing it with a 36 tooth gear when I put in the 2.56 highway gears. I have 26" tires, so one less factor. It worked great for me, and you should be able to nail it and have an accurate speedometer!!!


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## likethat (Oct 5, 2007)

700r4 .75 OD
200r4 .62 OD
The 700r4 has a lower 1st gear, so you don't need as much rear end gear as in a 200r4. The 200r4 is a straight replacement for a 350 turbo. It has the same yoke and the only mod is to move back the cross member and drill some holes in the frame. Then a new hole for the trans mount. 200 or 700 will need a converter lock switch for part throttle. The 700 has the same out put as a 400 so it uses the same yoke as a 400. It needs a trans adapter and the drive shaft shortened. As long as your not hooking the tires a 200 will live fine, but if your going to beat on the car, I would go 4l80. If you have to spend money then add a 6l80 6 speed and be super cool with double over drive:seeya


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## xconcepts (Jan 4, 2009)

So does anyone know why I'm getting 2.75 rotations from a 3.55 gear? 3.55 is verified.


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## Rukee (Feb 8, 2007)

xconcepts said:


> So does anyone know why I'm getting 2.75 rotations from a 3.55 gear? 3.55 is verified.


When your turning the driveshaft, is one wheel or both wheels turning off the ground??


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## xconcepts (Jan 4, 2009)

Both, the whole car actually, lol


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## jetstang (Nov 5, 2008)

xconcepts said:


> Both, the whole car actually, lol


SO, you are just rolling the car and counting revolutions? If it's not a posi, some of the turns may get lost in the spider gears. For a single spin rear, you have to hold one side solid, then spin the other side. When I did that, I came out with 1.5 because the spider gears ate up half the revolutions. If you counted teath, trust your count and don't try to figure it out, because it doesn't work for non posi's.


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## Rukee (Feb 8, 2007)

jetstang said:


> SO, you are just rolling the car and counting revolutions? If it's not a posi, some of the turns may get lost in the spider gears. * For a single spin rear, you have to hold one side solid, then spin the other side.* When I did that, I came out with 1.5 because the spider gears ate up half the revolutions. If you counted teath, trust your count and don't try to figure it out, because it doesn't work for non posi's.


If you do it this way, turn the 1 tire 2 complete revolutions and count the driveshaft turns for the ratio.


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## geeteeohguy (Feb 2, 2008)

If 3.55 is verified, why on earth are you screwing around counting the driveshaft rotations? Just drive the car!!!


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## xconcepts (Jan 4, 2009)

lol, I am just confused on why I'm not getting the correct rotations. And I'm 5mph off, so at the correct 65 I'm at 3K


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## geeteeohguy (Feb 2, 2008)

With stock size tires (about 26 inches) with a 3.55 rear gear, you will be right at 65 mph at 3000 rpm. If you go to a 28 inch tire, you will be at 70+ mph at 3000 rpm. Not a bad deal at all. Who needs an overdrive? If it's a stick car, you're right where you should be.


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## Rukee (Feb 8, 2007)

That's about right, @ 75mph I'm at 3200rpm with 295/50/15s and 3.55s with a stick.


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## geeteeohguy (Feb 2, 2008)

Rukee, I'm right at 3000 rpm at 75 actual mph with 27 inch tires and 3.36's with a stick. Pretty close....and about right where you want to be with a manual trans car. At 3000-3200 rpm, when you kick open the outer carbs, it feels like you downshift about 2 gears!!!! If you were running stock tires, you'd be closer to 3500 rpm @ 75mph. Bigger tires are a definite cheap overdrive! I remember running shorter than stock fat bias ply's way back when with 3.55 gears, and man, it was screaming on the freeway. But very, very quick in town.... Lot's of combinations to play with.


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