I have a04 f150 FX4 and I had the factory 18" rims with factory size tires on it. Well yesterday I put factory 20" with factory pirelli scorpion tires on it and Im trying to get the tire size programmed in my tuner and I did the formula by marking the tire and rolling on complete rotation and measuring it and it measured 98" but not sure I did it right..has anybody elese ran into this?? How do I do this ???????

 

If you do the measurement, you need to convert to metric. MM. If you use the formula in the manual, it already is in metric. Also, if you use the formula, deduct about 5% for the squish factor.

 

Got ya! I think I multiplied 98x25.4 and thats all I do and minus 5%?

 

Yes, the squish can be as little as 3%. If you can check it with a GPS, you can get it right on.

 

Alright..I appreciate your help

 

If you went from stock 18's(275/65r18) to stock 20's(275/55r20) that's called plus 2 sizing and no adjustments are needed.
Try 2479 or 2484.

 

Good catch. I didn't pay any attention to the sizes. There will be about .4mph at 65 mph difference. Pretty much dead on.

 

Thanx Bluejay!
I've been learning from you and Jack.

 

I think we're all learning from each other. I didn't catch the "plus 2" sizing (no adjustment required) point either.

The slight difference in speed has to be the fact that only the rubber "squishes", the wheel itself doesn't.

- Jack

 

Remember over on php's forum you and I had this discussion on "tg 150" question. Where with out squish figured in it a 1/2" difference but figure squish in @ 5 percent and it equalizes. "Ford's built in squish factor".