critter

iTrader: (9)

Finally managed to grab a file (took 14 min - is that normal?) and started to play with ls1edit. Mainly want to fix the speedo at the moment 'cuz of the 4.11 gears & Nittos. The pcm seems to think the tire is 3.5% smaller than it is - 25.29 instead of 26.18. Is this normal? Is it a fudge for rolling radius being less than diameter / 2?

NoGo

iTrader: (2)

Yup, that's normal.

Going by the tire diameter calc it comes out to be slightly larger than what the PCM has.

Personally, I measure the rolling distance and back-calculate to the diameter. Seems to give the best results.

critter

iTrader: (9)

I guess I expected that, 'cuz that is how we used to do it, but everything I've seen recently ignored it
Yeah, that sounds like the plan. I can't believe 0.83" of squash.

Rhino

iTrader: (6)

After looking at my KDW's this evening (at proper pressure) I wouldn't doubt that there's 3/4" of squash. There's quite a bit of sidewall bowing at the bottom of tire.

jimmyblue

iTrader: (11)

Squash might mess with your diameter measurement
depending on where you measure, but there's no
getting around the circumference (heh) - it never
changes, or only as much as you burn off. The belt plies don't stretch on street tires. NoGo's
method seems the right way to get the number.

critter

iTrader: (9)

I was looking at Nittos. The Goodyears do squash a bit. Anyway,
I get 25.22" actual vs 25.68" nominal for 315/35/17 Nittos, or 0.46" squash. Short sidewalls perhaps.