davidmoore1988

Hello,
I just bought a 1984 Camaro Z28.
It came with 245/40ZR18 tires. I believe that the rear end may have been upgraded at one time.
I notice that when I am on the highway the speedometer can be out as much as 40km/h. If i am in the city it can be out around 20km/h.
Is there a way I can fix that? Or am I stuck with it? (unless I want to revert back to the original tire size and rear end of course)
Also does anyone know the factory tire size for this car? The VIN number section in the door was taken off on a body job.....
Thanks

sofakingdom

iTrader: (1)

Observe the ACTUAL rear gear ratio, MEASURE the ACTUAL tire radius by using a measuring tape from center of wheel to ground NOT "calculate" by the trade size printed on the tire, install the correct speedometer gears to match.

Original tire size was called 215/65-15.

Note incidentally, the emphasis on words like "observe", "actual" and "measure". These are critical concepts. What the previous owner told you, what the RPO codes say, "calculating" the tire diameter from those stupid numbers printed on them, are WORTHLESS. LOOK AT what is REALLY there, MEASURE things.

davidmoore1988

Thanks for the tip. Install the correct speedometer gear to match the gear ratio do I need to tear apart the transmission?

Tuned Performance

iTrader: (92)

Depending if the drive or driven gear needs to be replaced.
the drive gear you need to remove the torque arm and driveshaft to remove the tail housing. The driven gear comes out of the tailshaft housing.

https://www.tciauto.com/speedometer-gear-calculator

davidmoore1988

how would I determine which gear needs to be replaced?

Tuned Performance

iTrader: (92)

You need to determine what you have first. Rear gear ratio, tire size see the calculator.
remover the driven gear and shine a light in to see what drive gear you have.

sofakingdom

iTrader: (1)

EFFF the "calculator" for tire size. It will be WRONG. (that is, it will NOT match the ACTUAL size of the tires... see comments about "observe", "actual", and "measure" above) Guaranteed.

MEASURE the tire/wheel radius from the ground to the center of the wheel with a MEASURING TAPE.

jmd

iTrader: (4)

245/40R18 are pretty close in circumference to 245/50R16 that came stock on these cars.
I very much prefer copying what GM put in these cars for drive gear / speedo gear combinations.

At this point, grab your a magnet, light, small socket set and pop the driven gear and sleeve out of the tailhousing.Cable can generally stay attached. Count the drive teeth, and count the driven teeth.

The 84 had a SPID sticker, I think on the console. Maybe you still have it. If so, the rear gear ratio codes are listed here: http://sethirdgen.org/reardata.htm If your RPO (Let's say GU5, 3.23) and your RPM (Let's say 3200 rpm) at 60 in direct gear (4th in a 5 spd or D in the 700-R4) are a 1:1000 ratio, you'll know your rear gear is close to the RPO. A rear differential lube change is a pretty cheap and simple job to really check what ring and pinion teeth counts are in the diff. and things may be off enough you just gotta do it.

Once you know what is really in the tailhousing, and the diff, you can match up what gears are needed.

Here are examples of what GM used: https://www.thirdgen.org/forums/tran...ml#post5765491 (Post 16)

https://www.thirdgen.org/forums/tran...ive-gears.html (attachments.)

So you're missing some gear teeth counts. And you may or may not have what you think you have, and you may as well verify before ordering anything yet.