Well I was looking at my stock tune and it showed my tire size as 29.06, but when I put in my actual tire size the size was 30.06, basically an inch off. When I bought the truck the dealer put 20's on it then another guy I bought so stockers from supposedly changed that value with a TechII to the correct tire size for the stockers.

My question is, does EFI Live do a good job calculating tire size if I put in the tire specs (255/70/16)? Perhaps I will just break out my GPS for my laptop to verify the changes.

 

final actual tire size will vary a little from one manufacturer to the next. The number you are looking for will also vary depending on the vehicle. The number you want input as your tire size will be measured from the ground to the center point of the wheel (while on the vehicle at rest) X 2. This is more accurate than measuring the entire tire, which doesn't account for sidewall flex. My speedometer is dead on according to my GPS.

 

I was wondering about factoring in the tire buldge, but even if the tire buldges some, yes that would change the tire height itself from the ground but IMO that would not change the number of revolutions per mile. Am I wrong?

 

i have to get my gps out to get my speedo just right...if i just use tire size calculator i'm about 2-3mph off

 

Your best bet for getting your speedo accurate is to check with GPS. EFILive has a utility for setting speedo accuracy based on GPS feedback. It's in the "Speedo Fine Tuning" tab in the speedo calculator.

 

Thanks Justin, I just checked that out, looks like I gotta get out the gps when I get home. I appreaciate the responses guys!

 

no, because the bulge is part of the rolling diameter of the tire, the "top" of the tire is meaningless because it's shorter as the tire rolls over it, hence, take the radius center of wheel to ground X 2

 

actually, not to be a dick, but measuring either of the radii would be wrong, the top is too tall, the bottom is too short. why not use a piece of chalk, i used blue chalk line, to mark the tire then drive it a complete revolution. this will create two lines on the ground that you can measure give you the circumference of the tire. i think this is what is important because it will be the same no matter what the tire pressure is. then just divide the circumference by pie or 3.14 if you want the diameter.

but then again theres a couple ways to skin this cat

 

I've only changed tire size once on a friend's truck, but I did it with chalk. Do a roll out with stock tires, and again with new tires. Divide them to get a proportion, and multiply by your current pulses per mile.