# 1970 GTO: Why are RPM so high when speed is low?



## TheBestVMan (Apr 19, 2010)

Hi All,

I have a 1970 GTO convertible which has a non-original Pontiac 400 (1972), a non-original M20 (1971) transmission and the original 3.56 posi rear. The car drives and shifts fine. It was originally an Automatic and was converted years ago to a Manual. I don't think the drive shaft was changed. 

My problem is that the PRM's get high very quickly in each gear. At 60 mph (4th gear) my RPM's are screaming at over 4000. The only solution I have read and heard is to lower the rear gears even though a 3:56 was considered mid range by Pontiac when the car was built. 

Any ideas on where the problem could be, and what the possible solution would be, will be greatly appreciated.


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## BearGFR (Aug 25, 2008)

4000 rpm at 60 mph with 3.56's sounds very high to me. In m 69, I've got 3.50's with 26-inch tall tires, and 70 mph is around 3400 rpm, and that's with a 3000+ rpm stall converter.

Here's a list of things to check, sort of in order by what I suspect is most likely to least likely.
1) You've really got gearing that's really a lot lower (numerically higher) than 3.56
2) Your tach is way off
3) You've got really tiny tires
4) You've got a badly slipping clutch


Bear


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## TheBestVMan (Apr 19, 2010)

Thanks for the reply Bear,

I believe the rear gearing is correct based on the casting #'s, but plan to pull the axle cover and check the actual gear's soon to be sure. 

The tach was rebuilt a few years ago and I believe it is close to accurate. Regardless, when doing 60 mph I can hear the engine screaming. I run 15' tires with 255 on the back. I had a trans mechanic drive the car and he thinks the clutch is OK and I never feel it slip.

I suspect the problem may have began when the trans was converted from auto to manual. Is this possible? Perhaps the drive shaft re-installed is too short?


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## ppurfield001 (Jan 21, 2008)

TheBestVMan said:


> Hi All,
> 
> I have a 1970 GTO convertible which has a non-original Pontiac 400 (1972), a non-original M20 (1971) transmission and the original 3.56 posi rear. The car drives and shifts fine. It was originally an Automatic and was converted years ago to a Manual. I don't think the drive shaft was changed.
> 
> ...



Are you sure you don't have an M-22 (rock crusher) that might be geared such that your RPMs are too high? I had an M-20 in my '67 GTO and it hit about 3 grand at 60 - 70 MPH with a 3:90 to 1 rear gear.


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## BearGFR (Aug 25, 2008)

TheBestVMan said:


> Thanks for the reply Bear,
> 
> I believe the rear gearing is correct based on the casting #'s, but plan to pull the axle cover and check the actual gear's soon to be sure.
> 
> ...


There's not a driveshaft problem I know of that can make it act like what you're describing. Keep in mind, that unless you're the orignal owner there's no quarantee that the actual gears inside the housing match the codes that are stamped on it. It's quite possible that an owner previous to you changed out the gears for something lower - I'd say that's even likely, since due to the transmission conversion.

You say it's a posi? Put it in neutral and up on jackstands so that both rear wheels are free to spin. Put a tape mark on one tire and also on the floor, get underneath and turn the driveshaft by hand, counting how many driveshaft revolutions are required to get exactly one tire revolution. That'll prove it pretty quickly.

Bear


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## ALKYGTO (Mar 29, 2010)

:agree 


Also, all 3 speed Automatic, 3 speed manual and 4 speed manual tranny's are 1 to 1 in the top gear, meaning input and output shaft rpm should be equal. I say "should" because unless you have a modern automatic with a lockup converter there will be some slippage in the converter.

Honestly, you would feel (smell :lol a slipping clutch. I say you've either got some 4:56 gears (GM made 3:55's) not 3:56 gears. I'll bet she gets with it to 60 mph, huh? 3:55's are about the perfect gear for cruising and acceleration.


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## geeteeohguy (Feb 2, 2008)

I'm with Alky on this one. All non od trannies have a 1:1 top gear. Automatics will rev 200-400 rpm higher due to converter slippage. Stock Pontiac ratio was 3.55, not 3.56. Regardless, if the clutch is not slipping and the speedo is accurate (verify with a GPS), you have some 4.11-4.33-4.56 gears out back. An ideal rear gear ratio for your ride would be a 3.23 or a 3.36. 3.36 gears will work in your present carrier assembly. Good luck.


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## TheBestVMan (Apr 19, 2010)

I've had the car on stands in neutral and did the wheel spin test with a mechanic friend, It can up as 3:55 on the nose. That's why I don't think the problem is with the rear gears.

Any other ideas? Would low compression heads like the 4X I currently have matter? The current engine is probably only putting out about 225 hp.

I'm planning to replace the engine with a stroked 420hp (1969 Pontiac 400) engine. Since 4th gear in the trans is 1 to 1, I assume I'll have the same problem with the new engine.


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## geeteeohguy (Feb 2, 2008)

You assume correctly. You have a gearing problem, not an engine problem. I would pull the cover off and count the ting gear teeth, and the pinion teeth, and do some division. Or easier yet, read the tooth count on the ring gear. My money say's you have a 4 series gear in back. With stock tires and a 3.55, you should be at 3400 rpm at 70 mph. 4000 rpm would be over 80 mph. 3.36 gears would be about 3000 at 70, and so on. If the clutch isn't slipping, the ONLY thing that it could be is your gearing. Are you sure you're in 4th gear with the trans??


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## TheBestVMan (Apr 19, 2010)

I will pull the axle cover and confirm the gearing.

Thanks to all for your opinions and input.


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## BearGFR (Aug 25, 2008)

Which tires are you running on the rear?
Have you verified the tach isn't lying?

Bear


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## GTO JUDGE (May 14, 2005)

If the tach is racing but the motor isn't it may be electronic in nature. Seems to me the engine would be screaming if the tach is up.


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## 68greengoat (Sep 15, 2005)

TheBestVMan said:


> Thanks for the reply Bear,
> 
> I believe the rear gearing is correct based on the casting #'s, but plan to pull the axle cover and check the actual gear's soon to be sure.
> 
> ...


Those tires should add 8 mph to your speedometer provided it hasn't been recalibrated with a different speedo gear. I run 255/60/15. When my speedometer reads 52, my actual speed is 60 per GPS. A taller tire will make the speed difference even greater, say 255/70/15.


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## TheBestVMan (Apr 19, 2010)

I'm going less off the tach and speedo and more off my observations. When on the highway I need to stay in the right lane because I can see the car is slower then the normal flow of traffic. It feels about 60 mph in 4th gear, no faster. And I hear the engine screaming loudly so I know the RPM's are high and I worry about doing major engine damage. 

I've done the tire spin test and the it came up 3:55. Once I eyeball the rear gears and confirm the actual teeth count and #'s on the gears are 3:55, as I believe they will, then I will out of ideas. 

Will post result as I learn more.


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## geeteeohguy (Feb 2, 2008)

I've driven 3.55 cars with stock height tires, and 60mph is about it for comfortable cruising speed. But they don't rev to death at that speed. That happens at 70 mph. Pull the cover and look...you'll probably see a set of FOUR-fifty-six gears instead of the 3.56 you were thinking of..


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## BearGFR (Aug 25, 2008)

TheBestVMan said:


> I'm going less off the tach and speedo and more off my observations. When on the highway I need to stay in the right lane because I can see the car is slower then the normal flow of traffic. It feels about 60 mph in 4th gear, no faster. And I hear the engine screaming loudly so I know the RPM's are high and I worry about doing major engine damage.
> 
> I've done the tire spin test and the it came up 3:55. Once I eyeball the rear gears and confirm the actual teeth count and #'s on the gears are 3:55, as I believe they will, then I will out of ideas.
> 
> Will post result as I learn more.


Before we get too far afield and have you dismantling your car... I know you're running 15-inch 255's, but 255-what? 70's?, 60's?, 50's? --- and when you say things like "60 mph... 4000 rpm... screaming..." are those actual speedometer and tach readings, or just what it "feels like"?

I ask because when I got the Beast together and started driving it, it sure felt like the motor was sort of 'running away'. In reality once I thought about it, it'd been many years since I drove a car that didn't have some sort of overdrive transmission and I just wasn't used to the sensation of an engine turning over 3000 rpm at highway speeds any longer. That used to be "normal" before the advent of overdrive transmissions.

Bear


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## geeteeohguy (Feb 2, 2008)

I agree...verify actual speed and actual RPM. Also, when I used to drive GTO's with a 3.55 rear, it wasn't too bad, as the National speed limit was 55 mph. "55 Saves Lives"!! When it went back up to normal years ago, the 55-60 cruising ability of the 3.55 gears could no longer hack it at 70-80 mph, or 85-90, which is where most of the pickups and Hondas drive at in my area. If you want to keep up with the commuters, you need a 3.36, 3.23, 3.08, or a 2 series gear if you have an automatic and drive on the highway. I agree also that we've been 'spoiled' by overdrive in virtually all of our cars for the past 25 years or so, and to the uninitiated, 3000 rpm seems awful high compared to 1850rpm running down the interstate............I still think you have 4-series gears out back.....


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## TheBestVMan (Apr 19, 2010)

Hi All,

I pulled the rear axle cover and checked gear codes. They read 39/11 which confirms 3:55. I also counted teeth to be sure. Gears look good. Almost too good. They might have been changed 10 years ago when I bought the car. The tires I run are 255/60/15.

When driving the car, I have to move through the gears quickly as the engine begins to race. I can hit 4th gear as slow as 30mph and the car runs smooth.

Any new ideas as to why my RPM's run so high would be appreciated.


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## 68greengoat (Sep 15, 2005)

As mentioned earlier, those are the same tires I run which will put your speedometer 8mph off. When you're speedometer reads 60, your actual speed is 68. Which will raise the RPMs.

As mentioned by others, you need to post actual RPM readings at actual MPH. Not the MPH that your speedometer reads. "Unless" your speedometer has been recalibrated. That way these guys can help determine if there is an issue or not.....


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## BearGFR (Aug 25, 2008)

TheBestVMan said:


> Hi All,
> 
> I pulled the rear axle cover and checked gear codes. They read 39/11 which confirms 3:55. I also counted teeth to be sure. Gears look good. Almost too good. They might have been changed 10 years ago when I bought the car. The tires I run are 255/60/15.
> 
> ...


We've given you lots of "ideas", but we really can't go any farther unless we're working with accurate tachometer and speedometer readings. Subjective feelings/observations can be very misleading.

Bear


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## Instg8ter (Sep 28, 2010)

punch up the GPS on your I-phone (they have an app for that) and take a ride that will tell you your speed and if your tach is working correctly will give you a baseline to start with.


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## fasterfiero (Sep 6, 2011)

jack the rear off the ground, mark flywheel & driveshaft, put in 4th gear turn the flywheel
one revolution and make sure you get one revolution on the drive shaft...... if so, I would say the clutch is slipping....


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## TheBestVMan (Apr 19, 2010)

Still working on the problem, but I appreciate the ideas. I like the IPhone app to check if tach PRM's are correct. Will provide update when I know more.


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