Dalannex

I have an 81', automatic with a 2.87 rear axle ratio. I know how to decipher tire size in that the first number is tread width in metric, the next is sidewall height in a percentage of width, the third is wheel size. The car had 255/60-15's on it when I got it. I had a spring sag problem kind of thing for awhile so when I put tires on I put 245/60-15's on to make the gap between the tire and the fender lip look better. (didn't like the lowrider look). Since then I did the VBP thing and am now thinking tires again. I like the front and am leaving those two alone. The rears need to be taller for two reasons, one is that the speedometer reads faster than I'm actually going, two is I need all of the RPM's I can get, I am running higher RPM's at 80 than I would like to be. Now, here are the questions:
1. According to my calculations a 245/60 is 9.64" wide and 5.78" tall, correct?????
2. I think my speedometer is reading 9 MPH faster than it should be, (speedo says 74, I'm actually going 65) does this sound correct considering my axle ratio and the fact that my tires are supposed to be 255/60's?????
3. A taller tire will drop my RPM's a little bit, and bring my speedo back closer to where it should be??????

I am thinking of going with a 255/60, which would be almost 1/4" taller than the 245/60's I have now, or a 245/65 which would be about 1/2" taller than what I have now. The 245/65's would mean my speedo would be off 9 MPH in the other direction, correct, because I am now 1/4" smaller than I am supposed to be? The reason i am thinking the 245's is that the VBP rear spring seems to be kind of close to the wheel as it is now, so i don't know how wide I want to go, as well, as the extra 1/4" of height in the 245/65's would help me to drop my RPM's. The speedo being off isn't that bad as long as I know how far off it is.. I can live with that. Anybody care to help me out, or at least give me your opinion on the subject????? :seeya

-Justin

hubes

Sounds like you already have it figured out, what do you need help with?

Larry B.

There is a small nylon gear in the transmission where the speedo cable hooks up you can get them in various teeth to correct a speedo error. It is very easy to do you need to take your out and count the teeth and then replace it with one that has the same percentage more or less that you already have. :auto:

79VetteMike

:iagree:

Dalannex

Just making sure I got everything correct. :thumbs:

Maybe I will change the speedo gear after I get my new rubber. So I just find out how far my speedo is off, calculate what percentage of the total speedo range that is, then buy a gear that has more teeth if my speedo is reading too slow???? Fewer teeth if it's reading too fast???? Is that correct???

-Justin

Paul L

Here is my thread on that subject.
http://forums.corvetteforum.com/zero...=598778&page=1

This site is an excellent reference complete with GM gear part numbers.
http://members.core.com/~faldrich/speedo.htm


[Modified by paul67, 10:05 AM 7/26/2003]