I have a custom tune from one of the popular mailorder tuners, but my speedometer is off. I've tried asking the tuner this several times, and I never get an answer back. So I appeal to the smart ones here...

I am having my CASE relearn done this weekend with a Tech2 when my buddy visits me. The one thing I really need an answer to: Can I use the Tech2 to adjust my PCM's tire size without affecting the tune?

My tune is suppose to have a 265/75R16 tiresize (31.65in diameter according to the tuner). My tires, inflated to 32psi currently, measured a roll of 95.5in so a 30.40in diameter (a 4% difference).

My speed testing via a GPS yields these highly unscientific numbers:
GPS vs Speedo
35 = 37 (5.7%)
50 = 53 (6%)
55 = 57.5 (4.5%)
60 = 63.5 (5.8%)
70 = 73 (4.3%)
75 = 79 (5.3%)
80 = 85 (6.3%)
Average error seems to be about 5.4% off.

From the online calculators, it seems like I should set the truck to think I have 265/70R16 tires, which are 30.6in diameter tires. A lot closer to what my actual tires are.
Thanks for your help!

 

I had the same problem. I did as you have done and rolled the tire. I then calculated the tire size and my speed and abs worked as it should.

 

Your going to have no luck with the TechII... Your tunner is somwhat right as its a 31.6 tire, But your measurements are proper and the correct way to do it. You just need to give him these numbers, Should be a simple fix.

 

Thank you guys, I'll use the Tech2 to do the CASE relearn and leave the tire setting alone. I will get in touch with the tuner and arrange to send the PCM back for an adjustment. I now have 93 octane available at my local station, so I can have the tune adjusted from an 89/91 to an 89/93 tune as well. At least I haven't installed the '05 EFans yet, because my stock PCM wouldn't be able to control them.

I'm just surprised that my tires are that far off on the diameter, despite them having plenty of tread left on them. Maybe I should pick a diameter that is halfway between new and worn out and have the PCM set for that. Then it will only be off by a little bit for the life of the tires, instead of spot-on when new and way off when worn.