928 obsession

After recently having a track day and driving my '82 and my father's '82, we both realized our cars have different gear ratios. In his car I had to shift a lot sooner than with mine. My car is a standard 928 while my father's is a comp package. Now the question is I know US 82's had the 2.73 ratio, but my car fells like it has a 2.20 rear end. Both transmission are G28 05's. In 1st gear his car redlines at around 44 mph, while mine redlines at around 53 mph. In 5th gear in my car I can cruise at 65 with the tac at 2000 rpm's, his hits 2000 rpms at around 52 in 5th. It gives the car a completely different feel. Is there a way I can determine what gears my car has? Were different ratios an option in 82? The reason I ask is I plan on either rebuilding my trans or buying a newer trans and I would like to keep the same gear ratios. Thanks

123quattro

Which rear end ratio did you prefer for the track?

danglerb

What is the full ID number of both transmissions? It should include any optional gear ratios.

Any big differences in tire size?

Are your speedometer and tach correct?

928 obsession

My transmission ID is G/28 05 11c 1411
I do not have my father's currenly on hand but I believe it was the same G/28 05 11c XXXX (except for lsd 220)
In terms of wheel size I have '16 and his has '17 wheels.
My gps and the speedo match. I have never checked the tach, but I believe it is pretty close to being dead on.

I perfer the 2.27 because I drive it daily, but the 2.73 was also nice. Probably if I drove my father's daily I would grow to like it, but it seemed the shift points were way to early to me.

Glenn Evans

Your original diff must have been replaced. The G28.05 only had a 2.73 (30/11) final drive ratio. Assuming that the speeds you have stated are via your GPS, the gearing of your father's car indicates that the overall diameter of his 17-inch wheels and tyres are somewhat smaller than the nominal outside diameter of 631.4mm (24.86 in) of the standard 225/50R16s, which will make the difference between the gearing of the two cars even more noticeable.

IcemanG17

it appears your 82 with the taller gears has the optional 2.27 rear end

"*81-83 S - 5-Speed Manual (Euro) 2.73 (2.27 - Optional) *81 from 1/14/81 Build Date "

are both USA models? Maybe the comp package allowed the optional euro taller rear end?

mark kibort

tire size wouldnt change the indicated speed vs RPM.

tachs are usually close, and speedos are all over the place , as we have discussed here. (generally, 3% showing faster than you really are )

If you are shifting at 10mph difference between the two cars, one is the 2.22 (like the '83 and '84 US) and the other is the stock 2.75 from '82.

Ive driven a lot with both and there are not much differences a the track. generally, its a match of the track, HP and driver ability. for example, I drove Scots 82 5 liter in a race with his 2.75. It was fine for Laguna seca and his power level. a 2.2 might have been better in some sections. My car is a 2.2 and it is more optimal for laguna with 75 more HP.

so, it really depends on a bunch of different factors.

mk

mark kibort

you cannot tell the difference with tires sizes based on the speedo. the speedo has no abilty to know if you have tractor tires or running on the hubs. a few MPH difference of actual speed, is way beyond the human speed receptors.

I run a 26" tire and its completely indistinquishable from the 25" tire I change to on occasion. And, of course, the speedo vs rpm doesnt change.

mk

Lizard928

If your fathers had a LSD factory then he would have head G28 05 /12 11c XXX

All G28 05 boxes have the same ratio from my research.
There is a comment from another person saying someone might have changed the diff.
The diff has NOTHING to do with the ratio. If the ratio had been changed this requires that the ring and pinion would be changed. The pinion is part of the layshaft. So you would have had to have the box fully disassembled and while it is unlikely, it is possible that it has been done in the past.

I prefer the 2.20 in 85+ cars as 1st with the 2.73 becomes almost useless. And the gearing works better all around. But it is really personal preferance.

karl ruiter

I have not found a lot of documentation about this, but I also had a '82 comp package and I am completely convinced the gearing was different than other 16v cars. I loved it and think the car felt much faster. First, as I recall was pretty short, but 2nd, 3rd, and 4th were very sweet.

mark kibort

I agree. Plus, with just a ring and pinon change, the gear ratios would be RADICALLY different, not just the 10%or so that the normal ratios compare between a 2.22 vs a 2.75/2.73. (besides the obvious direct comparison in 5th).

So, the question is, how do you have the same .-05 gear boxes and have them provide different indicated speeds in 5th gear at 2000rpm? It does almost sound like the ring and pinion have been changed if you see 65mph on one, but only 55mph vs the other. but, as you can see, that difference is only 15%, while a 2.2 vs 2.75 is near 20%. a 2.75 vs 2.54 would be 8%, so something is not jiving.

928 obsession

Yes, that is exactly how my father's comp package is. 1st is very short while the rest are taller. Still though my gears fell much taller than his. Just another test today in my car revealed that at 5200 rpm in 2nd is equilevent to around 73 mph. I'm not sure what my father's speed would be at that rpm range in seccond, but I'm sure it was much less, probably around 60ish. What is the speed at redline for 2nd gear for an 2.20 and 2.73?

mark kibort

2nd gear at 6500rpm is around 76mph.

2nd for a 2.73 is about 12% less, or near 63mph

all the gear boxes have the near the same spacing, besides 5th, which is quite a lot taller in a 2.2 than in a 2.75 (by exactly 2.2/2.75).

all you need to do is find the gear ratio chart and find out the differences of 2nd from the 2.2 or 2.26 vs the 2.75/2.73.

mk

76FJ55

here is a quick gear/speed calculator you can use.
Gear/Speed

when using 225/50x16s and 2000 RPM and 1:1 trany
you get ~67mph for a 2.2 rear and ~54 mph for a 2.73. these numbers are close to what you suggest in your initial post, so it is possible that you have a 2.2 and your dad has the 2.73 diff.

mark kibort

Sure looks that way. thats a 20% difference with the gear ratios and 20% difference in speeds using his numbers. (this would be for final drive only)