I installed my 17x9 WS6 wheels with 275/40/17 Kuhmo tires all around. With my stock 3.23 rear speedo was showing 5mph faster than I was going. Now I put a 3.42 rear and now my speedo is dead on. What gives? I figured that with the 3.42 rear I'd have a larger spread with the speedo showing faster.

 

Zephyr,

The 275-402 on 17" wheels should have been almost exactly the same diameter as the 245-50s on 16" wheels. If anything, mounting them on 9" wide wheels should have pinched the tread and made is larger, not smaller. That would make the true speed greater than the indicated speed, which is just the opposite of what you are saying, if I am understanding you correctly. Changing to a higher numerical gear ratio would tend to correct that situation, as is apparently now the case.

Are you sure the indicated speed was too HIGH with the 3.23 gears, or was it that the ACTUAL speed was too high?

Just askin'...

 

Well, when I had the 245/50/16's on the car with 3.23 gears, my speedo was correct. When I put the 17's on it was reading 5mph faster than I was going, example, speedo says 50 and I'm cruising 45.

BUT, tonight I found out that my friend who was pacing me the first time has an incorrect speedo, his is 10mph off. His speedo says 45 and he is actually doing 55.

So in reality, with the 17's and 3.23's, my speedo was 50 but I was actually doing 55. So now I have the 3.42's and my speedo is reading correct.

I am still a little confused, I may take it to a calibration shop or the Dyno and see if I am off, and if so, by how much.

 

What I like to do is find any cop radaring while stationary and pull over and have him/or her clock you on a fly by to see exactly what you are reading. Once you explain what you are wanting to do it for they are more than helpfull.

Man the last time I did it I got suprised by the local pd hottie,man did she look good. I got a smile on my face while I knew I was being clocked