Diablosport Forums

2003 Checy Silverado 2500HD 8.1L
I imported the Diablo tune to my truck and love it. How do I change the tire size so the speedo reeds correct.315/70/17---thx

hook up to your obd port... turn the key on, but no engine running... select the same tune you installed before... select Modify Tune... then select axle or gear ratio or tire size and you'll have the adjustment available in there...

enter your adjustment... press the center button on the predator to save the change, then press the ESC button a few times until it asks you "install tune now?"... select YES and it will install the tune with the adjustments you made...

U7193 purchased 1 week ago
Thanks it worked. The speedo is now only 2mph over instead of 8 under @ 75mph. Is it ok to try a tire size of 305/70/17 to try to dial it in even closer, even though its not the tru size of the tires (315-70-17). Also is the Diablo tune under performance the same as the 91 tune. I have the 87 and the Diablo tune but could not find the 91/93 tune. It has the latest revision.
Truck is 2003 Chevy Silverado 8.1L w/Ally trans. Thanks again for the help.

the Diablo tune is the 91/93 octane tune...

Ok sweet thanks for all your help & Is it ok to try a tire size of 305/70/17 to try to dial in the speedo even closer, even though its not the tru size of the tires (tire on truck 315-70-17).

Thanks for all the help Monte. I used the tire size of 285/70/17 & it is dead on at 0-80. I am going to do some logging and slope ajustments next. thx again.

cool deal man!!

Tires, Right Sizing: Park your vehicle in a safe area, on a level surface, and somewhere you can push the vehicle manually. With a piece of chalk, mark a rear tire, where the "rubber meets the road"; similarly, mark the road, where the "road meets the rubber"; that is, mark where the tire and road meet, say the six o'clock position! Hello! With the vehicle in neutral, safely, push the vehicle forward, until the chalk mark on the tire meets the road again; say 360 degrees or one full rotation. Again, mark the road at the new position! Finally, measure the distance between the two chalk marks on the road or pavement; voila! true tire diameter!

P.S. I don't care what the tire size reads; roll it out!

Is it something with the tire manufacturers not sizing the tires right or could it be an issue with the predator? I have 265/75/R16 (stock is 245/75/R16) and when I put that into the tune I am actually 4-5 mph slower than my speedo (checked with GPS at 70mph). I had to put in stock setting to get speedo accurate. My air pressure is right per manufacturer specs. I know that 265 isn't much different than 245, but putting in that info with the Predator shouldn't cause the speedo to be off that much at highway speeds.

I agree with mypostoffice. Roll it out to be accurate....

not a predator issue. Different brands have different nominal sizes and using the diameter method is also an issue because of sidewall squish. the OEM's use tire circumference when doing speedometer settings. That is the only proper way to do it.

That is why the roll out method is the best way to get a true circumference measurement of your tire setup.