conv90

My vette is a '90 convertible w/ AT
My AT related RPO codes says:
MD8 AT
G92 Perf. Axle ratio
GU2 2.73 Axle ratio
MX0 Autom trans w/ overdrive

some URL with RPO codes for corvettes says for ’90 corvettes G92 = perf. Axle ratio 3.07, some others URL with RPO codes says G92 = Perf. Axle ratio (without others addictions)

So I always thought I have a 2.73, because the G92 says my car has a performance rear axle ratio and the GU2 says the ratio is 2.73.

My speedometer says 69 mph when I’m in OD @ 2000 rpm

There are many free utilities on the web to calculate speed, rear axle ratio, rpm, Transmission Ratio, tire diameter….. ( ex: http://www.prestage.com/carmath/calc_gears.asp )

Using these free utilities (I found 3 or 4 or these utilities on the web) and using a 275/40/17 (25.66” diameter), with a 0.70 transmission ratio, and a 2000 rpm I have 80 mph with a 2.73 ratio and 71 mph with 3.07 ratio!!
My car is 69 mph @ 2000 rpm!!!!
I have a 3.07 rear axle ratio or more?? I’m confused…
When I purchased my car the dealer said that the AT was new. In effect if I look under the car, the transmission looks like new (it’s the only high polished part under the car with ‘MD8’ impressed on it.)

It’s possible that I have a new transmission with a 0.8 or 0.85 overdrive transmission ratio?
It exists a 700R4 with a ratio different than 0.70??
How to recognize??
Thanks for the patience an the long post.
-Beppe-

vader86

hmmm, i would say that you have a 3.07 rear from the 70mph@2000rpm because thats what i get from my 3.07. Try different mph and tell us the rpms youre running because 2.73 is still pretty close.

G92 is a pointless code, i dont even know why they put it on there as it means basically nothing, ive seen people with 2.73s that had it. I have 3.07 and i dont have G92.

There wasnt a 700r4 with a .8x:1, all were .7:1 to my knowledge. Unless this was a custom job, which i doubt.

conv90

How you know you have a 3.07? By RPO codes? You changed differential? Ther's some label or printed code on the case to recognize it?

vader86

by my RPO codes, its has its own RPO code G44, and its part of the Z51 option for my year


[Modified by vader86, 12:47 PM 12/5/2002]

dgoodhue

Your car originally came with 2.73 (which sound right for a 90 conv with g92 option), just make sure the RPO VIN number matches your VIN as I have seen lids swapped before.

Your speedo sounds like a 3.07's, but I would check it to see if you have the wrong speedo gears (and your speedo and odometer would be off) or someone swapped rear ends. The best way to determine your ratio, jack up the rear tires and mark and count how many times your driveshaft goes around for one revolution of the tires. A little over 3 is 3.07, 2 3/4 is 2.73's.

I would be shock if you have an OD of anything other than .7


[Modified by dgoodhue, 12:55 PM 12/5/2002]

Raistlin

I also saw in another thread a picture of the pumkin that had the gear ratio stamped on the case. You might check that.
Also, '90 tach's seem to be the most inaccurate of all the tach's. Mine reads high by 800rpm at idle to over 2000rpm at 4000rpm. If you have any datalogging equipment, you can check your actual rpm.

DON M

My car is a '90 roadster too. I have the same RPOs as you and the RPM at 70 mph is also 2000. I have had the same questions.
As far as anyone can tell me, G92 means that there is a non-standard ratio and you use the other axle code to determine the ratio installed. It seems like the coupes had the 3.07 and the verts 2.73. I still haven't looked under the car to find out.
I guess it is all mental masturbation and the car runs good either way.
(I wonder what the censor soft ware will do with the M word).

CFI-EFI

G92 IS a worthless code. All it means, is that the ratio installed in THAT car was the performance ratio for THAT car. Standard and performance ratios varied from year to year, with each engine, transmission, and even, body. Most available ratios were the"G92" of SOME car. As previously stated an excellent way of determining the ratio presently in your car is to jack it up and compare revolution counts. Also, as mentioned, the accuracy of the tach can skew your numbers. Not mentioned, is the tire ROLLING circumference. Tire sizing is a very inexact science. Rather than rely on a chart, mark your tire and the ground at a point where they meet. Then move the car one or more revolutions until the mark again meets the ground. Measure the distance between the marks and divide the distance between the marks by the number of revolutions you made. The more rev.'s made, the more accurate the out come. This can be carried too far. The dimention you end up with will be the distance your car travels per tire revolution. There are 5280 feet in a mile. The rest is simple arithmatic. And, NO, your 700R4 does not have a "special" overdrive ratio. It is 0.70:1 Good luck and...

RACE ON!!!

conv90

[/QUOTE]
Your speedo sounds like a 3.07's, but I would check it to see if you have the wrong speedo gears (and your speedo and odometer would be off) or someone swapped rear ends. The best way to determine your ratio, jack up the rear tires and mark and count how many times your driveshaft goes around for one revolution of the tires. A little over 3 is 3.07, 2 3/4 is 2.73's.
[/QUOTE]

I will try this..
the shifter must be in D? (1:1)
I'm editing this reply....
Thinking about my question... "...the shifter must be in D? (1:1) "
..I think I must do this test in N. Driveshaft and differential are linked by the ratio of the differential..... I'm Right?


[Modified by conv90, 9:12 AM 12/6/2002]

dgoodhue

It must be done in N so you can spin the driveshaft. The transmision gears down the motor, it doesn't affect the rear end ratio.