Jokerman

Well its in the interior, so i guess this works. Um ever since i bought my RS (june 2002) the speedo has been off. Its really noticeable past about 35mph. I know that at what my speedo say is just hitting 50 i am doing 45 and that 65 is about 58 or so. I am assuming the gearing is stock, and such, the tires are the correct overall height, ect. Is this just the speedo needing to be adjusted or is it morelikely there has been a gear swap? Just incase it doesn't show in my sig. its a 91 rs L03 auto. First half of 91 if it makes any differance.

Bandit5

It is either a gear change or tire diameter difference. Tell me what rear gears and the exact size tire you are running and I can find you the appropriate gears.

Jokerman

245/50/16. On a set of 85-87 irocs that came with the car.

Bandit5

I can't do anything without the rear end gears...

Jokerman

well the RPO reads as an auto car so they should be 2.73's. Then again it is 5 degrees so going out and cracking the rear end open to find out is not exactly a good idea.

kretos

my money is on the gears

Bandit5

Well, all I was able to calculate is the best gears for your speedometer. The optimum I have found is 20 drive and 42 driven. So, if your car has those speedo gears (or at least a very close ratio to it, i.e. 17 drive 36 driven) then i would say it is your rear gears. If they are differnet, just tell me what ratio they are, then I can find you better speedo gears.

89cmrodriver

Id bet money its your tires. The car came with 215/65/15's (unless u had the optional 16" wheels, if so disregard my whole post) Now you have 245/50/16's, they have a smaller diameter than the old tires (IIRC) which is making the speedo be off

Jokerman

89cmrodriver: Would the option of the 16inchers show in the rpo's? If not is there anyplace else on the car i can look to find out?

Bandit5: Thanks for the research. One question since you seem to be fairly knowledgeable on the subject, is there any way for the speedometer to get out of adjustment outside of the gears or is the speed differance way more on the gotta be the gears side?

Thanks all for the info and help, one of those nagging issues that i am trying to resolve.

Bandit5

The only thing I can think of would be a bad sending unit (sleeve). And maybe if a tooth broke on a speedo gear ( but I very much doubt they are broken, have never seen it yet). If it did originally come with 15" wheels, your new ones are only about 0.64" taller than your old ones. Although it doesn't seem like much, it will cause your speedometer to read slightly higher than your actual speed.

I am betting that it was the wheel and tire swap, the inacuracies seem consistant with that.

I would go to GM and buy a set of speedometer gears. Either buy the 20 tooth drive gear with 42 tooth driven gear, or ask them for the set that came with 16" wheels and 2.73 gear ratio. They aren't expensive, but they can be a PITA to install.

89cmrodriver

yes, check on the sticker on your driver side door, it will say what size tires belong on the car. If the sticker is gone, im pretty sure its a RPO code

mckenzie

i just bought a set of gears for my 91 z28. it came with 3.23 rear, but somebody put a 2.73 rear end in. well, for the same tires and rear....245-50-16 with 2.73 rear, it called for a 17 tooth red gear for the output shaft, and a 38 tooth light blue for the other gear on the speed sensor. it cost about $5 for each, or $10 for both. best of luck, tom