Tire size adjustment..

[F150 Man 4Eva]

Got my 87 Custom Perf loaded up and I went into custom options to fix my tire size and though I got it close...didn't get it quite right. I am showing about 2 mph higher then what my GPS says...so I over shot it a little bit. My question is...#1- would you worry about being off that little bit? #2- if I want to adjust it, can I go back into my custom options without going through the whole loading process again?

[shotgun]

This will answer your question re: Custom Options.

http://forum.gopowerhungry.com/faqs-gryphon-programmer/1101-q-about-gryphons-custom-options-menu.html#post9248

As far as speed being 2 mph off, it's really up to you. There are numerous posts and very good methods for determining the corrected tire size.

[F150 Man 4Eva]

Thanks a lot for that link..lots of useful info there. Seems like I am fine with being 2 off on my dash. Gryphon and GPS are "almost" dead on. I'll just leave it. Thanks again. And just fyi to all interested, 2718 is what worked for my 35x12.5x18's

[Jackpine]

That figure is just a teeny bit larger than what you'd get with a 3% "squish" compensation. Looks like a perfectly reasonable number to me.

- Jack

[Longshot270]

If you want to get it perfect clear the odometer on the truck and GPS, drive at least 10 miles then use this calculation:
Truck distance / GPS distance x tire size = new tire size
:thumbsup:
Also apparently Ford made the speedometer about 2 mph off.
My GPS, truck and gryphon all say the same thing.