Speed odometer off

[gmantheman]

When I purchased my 89tq tune from Doug, I had 3.55 gears and my tires were 255/50/17. Since then I changed my tires and gears. My tires are now 245/45/17 and I now have 4.10s. I modified my custom tune. I adjusted my tire revs to 785 per. min and gears to 4.09 (diablo doesn't have 4.10). When I compare my speed odometer to my GPS my speed odometer reads about 5mph than the GPS. When I modify my tune to 4.30 gears my speed odometer is off by only 2 mph. Does Doug need to re-send a new tune to reflect my new gears and tires?
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[DarkFireGT]

What tires do you have? Due to manufacturer variances, your calculation of 785 revs per mile could be quite off. I'm assuming you used the Miata calculator. If you go to the tire manufacturer's website, you can find the true revs per mile.

Diablo doesn't have 4.10 in there? That's goofy. If everything else is dead on, I would email Doug.

 

[gmantheman]

I used this formula (forgot where I found it) 20168/tire diameter. 20168/25.7 is approx 785

 

[DarkFireGT]

Ok, but again, what make and model are the tires? Tires differ between companies. Trust me. My 325/45/18 M&H's are not as tall as my 295/45/18 Nitto's.

 

[gmantheman]

BFG KDW

 

[DarkFireGT]

The revs per mile for the KDW is 810 for a 245/45/17. Try plugging that in and see what happens.

 

[gmantheman]

Thanks for the info. Just wondering, were did you get that information?

 

[gmantheman]

Never mind, it's right on their website.

 

[gmantheman]

I set the tire revs to 810, but still my speed odometer reads about 2 MPH. This is close enough for me. In fact aren't most speed odometers off by by -/+ 2-3 MPH?