Guys, this is really confusing me. I have a 92 Z28 5.0 TPI automatic. The RPO label has codes GU2 and G80.

According to the GM master RPO list, GU2 = 2.73 gears and G80 = limited slip. I can confirm it is limited slip as it spins both wheels.

Cruising on the highway at 75 mph, I'm at 2300 RPM in OD. However, all of the "gear ratio to speed" calculators online, such as this ( http://www.tremec.com/calculadora.php ) all give me the same result that is way off, saying 90 MPH:





Tires are 25", and I know 4th gear on the 700R4 is 0.7 ratio. BUT, if I change the axle ratio in that calculator to 3.23, it comes out just about right:





I got this car with VERY low miles, and in bone stock condition, so I highly doubt the rear end gears have been changed, but I don't know what else it could be. I verified the speedometer against a GPS and it's dead on.

Although I have a lot of experience working on cars, I'm new to thirdgens, and performance upgrades in general. So can one of you experts explain this to me?

 

A lot of people liked to swap gears in these cars even with low miles.. I remember a buddy of mine did his back in the day with 20k miles and I believe if you change the speedo gear your speedometer will read correctly..

I’ll have to plug mine into one of those calculators and see what comes up..

 

I like this calculator too, but it comes up similar to what you got with the other one..

https://www.chevellestuff.net/tools/rpm_calculator.htm

Its already setup for the 700r4

 

iirc, u can spin you back tires and count turns to confirm gear ratio

 

Lift the tires off the ground, put a piece of tape on the drive shaft and one of the wheels, put the trans in neutral, and spin the tire while counting the revolutions of the drive shaft. The more times you spin the tire around the more accurate your findings will be.

If both tires spin the same direction, then axle gear ratio is the drive shaft revolutions diveded by tire revolutions. If wheels spin in opposite directions, the calculation is slightly different.

 

Forget all the codes and counting turns, blah blah blah. Pull the cover off the back, turn the ring gear until you see a stamp with two numbers with a : between then. Divide the larger number by the smaller, and that's your gear ratio. Every other method you're guesstimating. Reading the manufacturer's stamping tells you the actual gear ratio.

 

Fascinating. I'm gonna try that this weekend, thanks!

I'm also questioning the accuracy of the tach now, after reading on this forum that these cars have a common issue with the tach reading too high, which is fixed by replacing the tachometer circuit board. Is there a way to test that? I have a Tech 1A scanner which should read the RPMs, but would that reading also be thrown off by a bad tach circuit board?

 

There's a really good solution to this - buy and install a 3.73 gear set - the're cheap. Whatever is in there will be lower than this so you'll get a performance boost no matter what. And if it still has the 2.73.... well then it needed swapped out didn't it?

Tach reading will not affect the ECM. ECM determines RPM from the distributor/coil. It could care less if the car even has a tach.

GD

 

Got it. I'm gonna physically check it tomorrow and find out what's in there. If it has a 3.23 I will probably keep that. This is not a daily around town car, I mostly do long highway cruises with it. And yeah, 2.73 is crazy for these cars.

 

Who knows when the rear fluid was changed. If ever. Verifying gears would be a good time to do that:

What Gear Ratio Do I Have?
Jerk the diff cover. The numbers are stamped on the ring and pinion.
41 : 15 = 2.73 (RPO GU2)
41 : 14 = 2.93 (RPO GU3)
40 : 13 = 3.08 (RPO GU4)
42 : 13 = 3.23 (RPO GU5) / Approx. RPM’s at 70 mph--2133
41 : 12 = 3.42 (RPO GU6) / Approx. RPM’s at 70 mph--2255
41 : 11 = 3.73 (RPO GT4) / Approx. RPM’s at 70 mph--2463 (Note that 3.73's were only available 1983-86)

JamesC

 

Ok so I just did this, and for each 1 revolution of the wheels, the driveshaft goes around 2 and 3/4 times approximately. So that would mean 2.73 gears right? That matches up with the RPO label, but then why do all those calculators say different?

I did change the rear axle fluid a few months ago, but didn't think to look for markings on the gears then.

 

So you know your tire diameter - 25" (Was this height calculated, measured, etc, probably close enough anyway)
You know your speedometer is accurate( (verified against your GPS)
Pull the diff cover and verify the tooth count against the gear markings (people make mistakes)
Assuming all is well in your transmission....
That should just leave your tachometer as the unknown

 

Turns out the tach is wrong. I connected my Tech 1a scan tool to the car and checked the RPM on there, and the tach is reading quite a bit higher. Apparently it is a common problem where a small circuit board on the tach gauge goes bad and does that, there are several long threads about it. Just ordered a new tach circuit board from Cajun Tach Shop.

 

Not to hijack this thread, but my rearend came out of a '84 Z-28 with the L-69 H.O. and 700R-4. From everything I read, it should have been a 3.42 ratio, which I thought would be ideal, but spinning the wheels and counting the driveshaft revolutions just wasn't coming out right, so I pulled the cover and counted the teeth on the ring & pinion. 'Turned out they are 3.73s. Apparently, this was the "G92" option. I thought 3.73s could be had only with a manual transmission. Anyway, counting the teeth removed all doubt, and the gear oil needed to be changed anyway.

On another note; I don't think those online calculators are 100% accurate, but probably somewhat close...