# What's Most Versatile Rear Axle Ratio For This Silver Dad?



## Sdpepper (Jun 3, 2021)

I'm a 70 year old guy enjoying my 67 GTO hardtop. My racing days are long past but I enjoy driving my car and hearing and feeling its power. That said, I learned my 4 speed has a 2.93 ratio and read several posts here recommending better options for people like me who seek a lower RPM on the highway, help engine run easier at freeway speeds, enjoy my oldie tunes driving, and perhaps cooler running temps, though my car doesn't run hot. I'd appreciate goat owners advice on what ratio to consider to achieve these things. My car has a WD axle code, runs 17 inch tires, a 400 ci engine, and runs at 4000 RPM's at 70 miles per hour on the freeway. Thanks.


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## O52 (Jan 27, 2019)

With a 2:93 you should be turning less than 3000 RPM. I suspect you probably have 3:90s or really small diameter tires.

Or an inaccurate tach


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## Sdpepper (Jun 3, 2021)

Thanks Ed. That’s interesting. My tires are 255/50/17 in the rear so not really small diameter. But, the tach being off is a possibility I hadn’t considered. I’m curious what other goat owners running a 2.93 measure on their tach at highway speeds.


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## Baaad65 (Aug 29, 2019)

I have a 2:98 first gear but your 4th gear should be 1:1, 3.42 rear gear with 28' tall tires and I'm turning the tach around 2400-2500 @ 60mph


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## Sdpepper (Jun 3, 2021)

Baaad65 said:


> I have a 2:98 first gear but your 4th gear should be 1:1, 3.42 rear gear with 28' tall tires and I'm turning the tach around 2400-2500 @ 60mph


According to Zazarine's book, 65's didn't come with 3.42 ratios so I suspect you changed out the original? If so, what was your original ratio and what was the tach reading?


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## Baaad65 (Aug 29, 2019)

Idk it was the original open 10 bolt, swapped it for a 12 bolt.


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## NOS Only (Nov 14, 2017)

I agree with the others. A 2.93 gear is a highway gear usually found on vehicles like cop cars. Dog off the line but top end is almost infinite. You can't trust a factory or cheap tach.


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## Sdpepper (Jun 3, 2021)

NOS Only said:


> I agree with the others. A 2.93 gear is a highway gear usually found on vehicles like cop cars. Dog off the line but top end is almost infinite. You can't trust a factory or cheap tach.


Thanks, NOS! It's a factory tach and likely needs recalibration.


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## Baaad65 (Aug 29, 2019)

Sdpepper said:


> Thanks, NOS! It's a factory tach and likely needs recalibration.


Sorry I misunderstood took it as you were saying your 4 speed trans had a 2.93 now I got it, my factory tach was off too where I thought the motor was quiting at 4500, come to find out I was hitting the rev limiter at 5800 ! The oil pressure gauge was off too, you want those two gauges definitely working right, so put in a whole new set of custom designed Speedhut gauges into the original dash...looks and functions great. Maybe you need a 3.23 or 3.31 ? I didn't want any more than a 3.42 and I did the rear end before I new the trans could have a 2.98 first gear otherwise I would have gone the next ratio down in a 12 bolt but it's sure fun coming out of the hole and I don't do long trips, just need to keep 28" tires on the back.


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## Sdpepper (Jun 3, 2021)

Baaad65 said:


> Sorry I misunderstood took it as you were saying your 4 speed trans had a 2.93 now I got it, my factory tach was off too where I thought the motor was quiting at 4500, come to find out I was hitting the rev limiter at 5800 ! The oil pressure gauge was off too, you want those two gauges definitely working right, so put in a whole new set of custom designed Speedhut gauges into the original dash...looks and functions great. Maybe you need a 3.23 or 3.31 ? I didn't want any more than a 3.42 and I did the rear end before I new the trans could have a 2.98 first gear otherwise I would have gone the next ratio down in a 12 bolt but it's sure fun coming out of the hole and I don't do long trips, just need to keep 28" tires on the back.


Thanks, Baaad. I’m definitely going to look into the tach accuracy. I’m I also right in thinking that if a 2.93 is a highway ratio, a 3.23 would result in a higher RPM while giving me a little more punch off the line?


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## O52 (Jan 27, 2019)

Yes.


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## Baaad65 (Aug 29, 2019)

Sdpepper said:


> Thanks, Baaad. I’m definitely going to look into the tach accuracy. I’m I also right in thinking that if a 2.93 is a highway ratio, a 3.23 would result in a higher RPM while giving me a little more punch off the line?


Yes but also depends on your first gear, do you have a wide or close ratio gear box I think they were 2.21 or 2.51, if 2.21 maybe you could use a 3.31gear even and taller tires will help keep your rpms down so idk if you stated the height but go to tiresize.com and it will give you all the information on tire size vs rpm.


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## PontiacJim (Dec 29, 2012)

Sdpepper said:


> According to Zazarine's book, 65's didn't come with 3.42 ratios so I suspect you changed out the original? If so, what was your original ratio and what was the tach reading?


Am a little confused on your numbers. Is the 2.93 the first gear ratio of your transmission or the rear axle gear?

Assume you mean rear gear ratio. *"WD" axle codes do begin in 1967 for the 2.93 *rear gear ratio.

The "standard" rear gear ratios for the 1967 GTO, depending on engine, ranged from 2.93 to 4.33, but the gear ratios from 2.93 to 4.33 could also be ordered as optional regardless of the engine selected.

P255/50/17 shows to have a 27" diameter.

Final trans gear ratio is 1:1

Your tach says 4,000 RPM @70MPH.

Let's throw out some RPM numbers at 70 MPH using available Pontiac gears.


2.93 gears - 2,552 RPM
3.08 gears - 2,683 RPM
3.23 gears - 2,813 RPM
3.36 gears - 2,926 RPM
3.55 gears - 3,092 RPM
3.90 gears - 3,397 RPM
4.33 gears - 3,771 RPM

4.56 gears - 3,972 RPM (non-Pontiac gears)

So you can see that you would need about a 4.56 gear to be revving 4,000 RPM @ 70 MPH. There would be no mistaking 4.56 gears as acceleration would be rocket ship fast.

Other alternatives - 
A.) Wrong speedometer gear in the transmission housing.
B.) Analog tachometer hooked up to an electronic ignition distributor.
C.) Tachometer is flat out wrong and needs a rebuild/calibration.

Best gear is your choice from above that puts the RPM where you feel it is comfortable. How often do you do 70 MPH? 2.93, if your car has that, is the best number from above. Typically, 3.08 is one of the lowest rear gears with a wide ratio Muncie M-20 2.56 first gear ratio manual trans, 2.93 would be a TH-400 automatic transmission gear. Selecting a taller tire would drop the RPM's, but with a manual trans, it could cause you to need to slip the clutch a lot more to get going from a stop and that means more clutch wear faster.

Changing gear ratio's can need a change of carrier that they bolt to. I can't recall what the series of gear ratio's are grouped into based on the rear end gear carrier. Going to anything more than 2.93 will help with acceleration, but push your RPM's up higher @ 70 MPH, use more gas, and put more wear on engine parts.

This is why many do a 5-speed Overdrive transmission conversion so you can keep better gears in the rear end and take advantage of the overdrive.


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## Baaad65 (Aug 29, 2019)

Sdpepper said:


> Thanks, Baaad. I’m definitely going to look into the tach accuracy. I’m I also right in thinking that if a 2.93 is a highway ratio, a 3.23 would result in a higher RPM while giving me a little more punch off the line?


You can also get a MPH app on your phone to verify while on a drive, works by GPS.


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## Sdpepper (Jun 3, 2021)

PontiacJim said:


> Am a little confused on your numbers. Is the 2.93 the first gear ratio of your transmission or the rear axle gear?
> 
> Assume you mean rear gear ratio. *"WD" axle codes do begin in 1967 for the 2.93 *rear gear ratio.
> 
> ...


Jim, this was very helpful. Can’t thank you, Baaad, and 052 for your meaningful responses to my post. Yes, my axle ratio is a 2.93 insofar as it coincides with the WD code and wasn’t changed by the previous owner. I will definitely check the validity of the tach as well as the two other possibilities you mentioned. The RPM data related to gear ratio certainly suggests the 2.93 should be a good fit for me given my driving habits. That said, I’m curious about the overdrive option sensing that when I’m going 70 a freewheeling gear would be nice to have. Again, I’m learning quite a bit since joining the forum and happy I did!


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## PontiacJim (Dec 29, 2012)

Sdpepper said:


> Jim, this was very helpful. Can’t thank you, Baaad, and 052 for your meaningful responses to my post. Yes, my axle ratio is a 2.93 insofar as it coincides with the WD code and wasn’t changed by the previous owner. I will definitely check the validity of the tach as well as the two other possibilities you mentioned. The RPM data related to gear ratio certainly suggests the 2.93 should be a good fit for me given my driving habits. That said, I’m curious about the overdrive option sensing that when I’m going 70 a freewheeling gear would be nice to have. Again, I’m learning quite a bit since joining the forum and happy I did!


The 5-speed overdrive transmission can be expensive and something a shop would do unless you have the shop & inclination to do the conversion. It may include cutting the floor pan and modifying the console if you have one.

So a better bet in your position might be the Gear Vendors add-on overdrive. Again, not inexpensive, but cheaper than a 5-speed OD swap, and easier to install as it bolts right up to the back of the transmission. Could be done in an afternoon. Biggest catch is shortening your driveshaft, but they give you the info as to what to do. You might just as easily purchase a new driveshaft to length and keep your original.

Installation is very simple in most applications as follows: Just remove the transmission extension housing and bolt on the *GEAR VENDORS* replacement housing which has an eight hole pattern to receive the overdrive. Tighten the eight nuts and take the driveshaft with the new yoke (included) to the local driveshaft shop to be shortened (as per the instructions). Plug in the electronics and fill the overdrive with the recommended lube, and you are ready to drive.









GM 4-Speed Manual - Gearvendors


GM Manual Transmissions Muncie M21/M22 (Also M20 and BW T-10) Gear Vendors owes a lot to the Muncie close ratio trans. These words are very similar for the M20 and T10 but with some slight ratio differences depending on model.All M21/22 have the same 22% upshift and 28% downshift spread as the...




www.gearvendors.com


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## Machinest-guy (Jul 19, 2019)

Good evening. I too am an old guy - will be 70 this next year. Don't know if my racing days are done, actually doubt it, but the conversation is about rear axle ratios. I've had my GTO since 1971 or so. Had four different gear ratios in it over the years with (mostly) the same Munci 4 speed (swapped M21 and M22 versions just for kicks for a couple of weeks once) OEM non rock crusher is in it now. I doubt my tachometer is accurate - never been serviced and always had some odd jump / bounce action that didn't seem fully right. So I can't speak to exact RPM vs speed. ( I use a gear ratio program on my computer for that as Pontiac Jim showed us). What I can say is I've tried ratios from 2.50 something to 4.50 something in Pontiac and Chevy 12 bolt flavors. 

Subjectively I never liked the 2.50ish gears. I could not get lazy about starting in second and rev'ing it a bit to drop into 4th. Way too much clutch slipping was needed to do that. The 2.5ish gears were ok on the freeway but my motor was never happy at comfortable freeway speeds. I always had to downshift to get moving. The 2.50ish gears were put into the car when I wore out the OEM 3.55 gears my car was built with. 

Those gears were nice. My car seems responsive and crisp with 3.55 gears. I could start in 2nd and wind it out a bit then drop into 4th to cruse along. I street raced and auto crossed with them. I traveled 10's of thousands of miles on them. They never seemed to do anything wrong when matching driving desires to conditions of use.

I'd built a 4.11 axle for my stock car which ended up in my GTO when I "quit" racing for a few years to crew chief and build engines. 4.11's were like night and day in acceleration. 4.11 gears really woke the car up and were good for only one thing. Rapid starts. Other than that they drove me crazy. First gear was useless. May have well not been in the box. On the freeway the engine was always running way over speed for any sense of comfort or economy. Then some 4.56's got in there and that was even worst. I put the 4.11's back in because I didn't have anything else at the time. It was the axle of last resort sort of situation because I needed to be able to move the car around or I would have just let it sit on stands.

I did try an Auto Gear replacement transmission which was a special built 4 speed Munci overdrive. They did that Munci modification by changing 3rd to be the over driven gear and changing the shift pattern making "3rd" become "4th". I figured out if I flipped the 3-4 shift arm 180 degrees it reverses the motion in the linkage so the shift pattern became normal. It was an interesting custom gear box but wasted behind 4.11 gears. Too low of a gear ratio is just too low, and no common over drive will fix that. Well maybe that isn't true. Gear Vendors had some very tall ratio units years ago I installed in trucks, but I'm not aware of current car offerings. I'd bought the custom Munci for a customers Firebird so had to give it up and install it in his car as promised. I tried to get Auto Gear to make a few more but no joy. I last tried to get another one about 10 years ago. The 4.11's bugged me so much my car was not fun so it went back into "storage mode" for a bunch of years while I worked on other things. 

A year ago I got in line at Currie Enterprises for a new axle assembly. I ordered their Ford 9 inch housing with an aluminum center and NASCAR guts in a 3.50 ratio with rear disc brakes. I had the driveline shop make a larger diameter custom driveshaft to match the HD yoke and so on. It took awhile to install the axle properly and overhaul my rear suspension - make everything pretty, re-engineer the rear shock mounts, etc. Stories to tell about rear shocks and suspension, but that's for another day, after I get an exhaust system back into the car and start driving it again. I believe for me, and my desire to just enjoy driving the car, 3.50 gears are the best choice.

I'm sure the fun will come right back, but I sure wish Auto Gear would make a few more of those modified Munci gear boxes. To me that would be an ultimate set up. These Pontiac GTO's have so much power they don't need a bunch of gears to go fast. Three under driven with one over driven ratio is plenty. I also feel changing a GM rear axle ratio is best done by changing an entire axle assembly because you end up with less "junk on the floor". And, unless it is a damaged unit, you have something which can be sold or stored reasonably easily. But a big reason to go the Ford 9 inch route is, it is even easier to change, because you deal with even fewer parts when swapping pre-built center sections. Ratio changes can be done pretty fast without messing with suspension issues and setting up gear sets. 

This are my considered opinions from many years of working with these cars. I hope it adds a voice to the other opinions expressed. Best of luck to you getting to the fun stuff. Ladd


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## 64since65 (Dec 11, 2019)

I agree with Machinist-guy (good evening young man) that something in the 3:50 range would be best. My thought would be anything from 3:23 to 3:55. I had a 3:90 axle in my '64 and replaced it with an axle that turned out to be the economy axle for an automatic trans (I think it was actually lower than 2:93 - maybe 2:78?) only because the 3:90 broke a bearing cap and I needed something quick and wasn't able to check adequately - and no internet to get help from at the time. Even with a wide ratio Muncie it was a real pain to get started in 1st gear - but I could go up to about 60 MPH before shifting. And, yeah, that low axle ratio would have resulted in a lot of clutch wear. The bad news about the really low axle ratio was that it didn't improve my gas mileage at all. Apparently I went "over the hump" and would get better mileage in that 3:23 to 3:55 range.

Strictly a best guess but I'd say 3:23 for better mileage and 3:55 for decent mileage (if anything is decent in those cars) with a bit more fun off the line.

And I wish I was still 70. Starting a rebuild at 72 has resulted in a recent delay due to falling and hurting my back. Maybe I should leave cutting down trees to the younger guys. I'm hoping to get back to work on the GTO in another 2-3 weeks.


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## Sdpepper (Jun 3, 2021)

64since65 said:


> I agree with Machinist-guy (good evening young man) that something in the 3:50 range would be best. My thought would be anything from 3:23 to 3:55. I had a 3:90 axle in my '64 and replaced it with an axle that turned out to be the economy axle for an automatic trans (I think it was actually lower than 2:93 - maybe 2:78?) only because the 3:90 broke a bearing cap and I needed something quick and wasn't able to check adequately - and no internet to get help from at the time. Even with a wide ratio Muncie it was a real pain to get started in 1st gear - but I could go up to about 60 MPH before shifting. And, yeah, that low axle ratio would have resulted in a lot of clutch wear. The bad news about the really low axle ratio was that it didn't improve my gas mileage at all. Apparently I went "over the hump" and would get better mileage in that 3:23 to 3:55 range.
> 
> Strictly a best guess but I'd say 3:23 for better mileage and 3:55 for decent mileage (if anything is decent in those cars) with a bit more fun off the line.
> 
> And I wish I was still 70. Starting a rebuild at 72 has resulted in a recent delay due to falling and hurting my back. Maybe I should leave cutting down trees to the younger guys. I'm hoping to get back to work on the GTO in another 2-3 weeks.


Thank you for sharing your experiences. I’ve heard others mention their preference for the 3.23 as an overall good ratio. I’m wondering if anyone has gone from a 2.93 to a 3.23 and what they noticed was the difference?


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