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?

 

I, too, have a Diablo tuner and it is an easy change to make. Keep adjusting the tire revs per minute til you get it spot-on. With my factory 235/50/18's I set mine at 754 revs per mile. On drag days with the 275/40/17 drag radials I have to bump it way up over 820 revs per mile (shorter tire revs more per mile).

 

I gave him the correct revs per mile for his tires, so he should have the problem solved. He hasn't responded yet whether or not it fixed it.

Just a heads up for anyone out there. When correcting your revs per mile, always go to the manufacturer's website and get the tire revs per mile from them, not a calculator. Due to variances in manufacturing, some tires go by the standard calculation, but many don't. The standard calculation says he should set it to 785, but the tire manufacturer actually says 810.

 

I set it to 810 after seeing the website; however it is still off by about 2 MPH. I am not going to worry about it too much.

 

Last night a Florida Highway Patrol Trooper calibrated mine for me. Mine was off by 4 mph. Had cruise set at 69, his radar said 73. Oops, Oh well, I now have another "Special Document" for my car.

 

I calibrate mine using the mile markers along I-5. I do a 10 mile run and see how much it is off, then adjust and go compare against the mile markers on the freeway again. After several passes against the mile markers, I have mine spot-on 100%. Math and calculators are good at getting you in the ball park, but real world measurements are always the most accurate way to set them.