I've been trying to set my speedometer based on tire size. i've measured from ground up using a level on the tire and it's 33.75 all around. If I set the programmer to 33.75, I'm ok until I get to 50mph then everything starts to be under by about 3mph. I've lowered the size in the programmer a 1/4" at a time and same results. I'm currently at 33.25 and still off by 2-3 mph as get to 60mph plus. Even if I adjust in the opposite direction, lower speeds ok, higher speeds off. Running 315/70/17 which measure 34.3" un-weighted. Any suggestions?

I'm using GPS to check against, same GPS I used to confirm accuracy when running stock and all speeds matched. - Thanks

 

I had the same issue when I had 315. You might need to play with tire pressure to get it close. I was never able to get exact but close enough. I now run 37's and have it dialed in on the money.

 

To me it sounds like the obvious solution is to get 37s!

 

That's another way to go!

 

well, 37's is an option but I guess I'll get close enough until then

 

Could be centrifugal force at high speed causes your tire to spread out beyond the standstill measure. This is especially true when tire size is more than double rim size.

 

Interesting!. I'm running stock rubi tires and I'm off about 5 mph over 60. I haven't measured it at lower speeds. Guess I'll have to "play" with tire size to see if I can get the correct speed.

 

Wouldn't you go up in tire size to bring the speedo speed up? Id just adjust it until it's dead-on at 60 and live with it being off down low.

 

no - it doesn"t help going higher, I had it set at 35.5 same results. I'd rather have the lower speeds - school zones - accurate and the higher speeds slightly off

 

Had the same issue and called superchips, told me it is still with in spec. Reason I called was when I put 35" instead of 34.75" I lost most of my dash displays, tack, speedo etc. it was bizarre. Went back to the 34.75" and everything went back to normal. They never did get back to me on what the issue was. And I was using the most current updates to the programmer since I just downloaded the updates and decided to change the tire size to get it a bit more accurate.

Now that I have a Diablo Programer, which I like lot's better, it still has the same issue with the speed calibration, but I haven't played with it yet as I just switched 2 weeks ago.