Took 20's off today to feel diff
 

SO today I realized it was nice and i took my 20's off for the winter and put my stock 17s back on , figured i would feel a hell of a difference since the rim is lighter and less wheel rotation, but it honestly felt slower, i dont know why, is it just me?

it felt slower really did you adjust the tune for the stock tires size

nah how do i do that with the diablo , do i need a new tune?

diablo's manual is totally wrong they say do 20168 divided by your tire diamater that is incredibly wrong, with my 20s the tire revs were at 715 so i figure 17s are at somewhere like 840

that's weird... you should have felt a nice gain...

well i know one thing for sure, I need a tire in the back, stock pirellis jsut spin lol,, all first and second

im taking my twenties off for winter too but i bought 18" fanblades and painted them black :) and sold my stocker 17's

The OE 235/55-17's run about 768 revs per mile (all of the OE sizes fall right around 770 give/take a tiny bit).

Just so you know, that 20168 isn't really accurate for determining the wheel revs per mile for "rubber" pneumatic tires, though it would be correct for something like a free-rolling steel railroad car wheel.

There is about a 3.5% factor that accounts for the tire tread compressing as it contacts the ground that Diablo is ignoring. This is a number based on tire tests, not something that can be determined mathematically. It is not the same as calculating based on the tire's loaded radius either. For most tires, 3.5% is a good number, though some of the Nittos appear to run closer to a 3.0% correction.

Where the Diablo manual tells you to do 20168 ÷ 29 = 695, the 3.5% correction gives you almost 720 (which should be in much closer agreement with the Specs pages at the Tire Rack).

Best to look up the tire specs for your make and model tire, and use those numbers directly (or as close as possible, maybe rounding up to the higher increment of 5 rather than down to account for gradual tire wear).

Next best (and should be close enough most of the time) is to bump up the Diablo math by 3.5%.


Whiteflash - you're running Nitto NT555's in 255/45-20? (at least those Nittos seem to run with a slightly lower 3.0% correction.) BTW, I agree with Tex that you should have felt the difference. Maybe try checking the moderate driving part throttle response instead of WOT just to keep wheelspin possibilities out of it.


Norm

Yeah, you're way off on that one. The stock tires (235/55-17) are 768 Revs Per Mile. Go back in, and re-set them properly.

Blast, beaten!

Yeah, but you did manage to sneak it in before I got any of the edits posted. ;)


Norm