I am running 285/55R20 BFG AT's on my 08 F150. Does anyone know what the calculation would be to program it in my edge programmer? Thanks for any help.

 

Sure - those tires are a little over 32" in diameter, making the circumference about 2580mm. But, that doesn't account for "squish" (the flattening of the time due to weight of the truck) that effectively reduces the rolling diameter. I've found that subtracting about 3% is about right so, 3% of 2580 is about 77 and 2580 - 77 = 2503mm. Give that number a try.

If the Edge speedometer reads high with this number, reduce it. If it reads low, increase it. The dash speedometer will always read about 3% higher than the Edge because Ford designed it that way. (You want the Edge to read correctly).

- Jack

 

thanks!! i had to take my truck in to ford this morning because my ac was having issues. i thought i better be on the safe side and take the edge out and when i was putting it back in i could not find my notes to what i had it set on.i stay on level 2 and i only adjust the speed limiter and for tire size,i figure i better leave the rest alone,don't want to mess anything up.hey jack, i also notice every now and then my truck kind of "sputters" that is the only way i can describe it,first i hear it in the exhaust (i have magnaflows)and then immediately i feel it. but it always clears up in just a few seconds.i have never had any codes pop up.any idea what that might be? once again, thank you for all your help, if you ever come to bradenton, i owe you lunch and a beer! -James

 

Believe it or not, the factory programming uses tire revolutions per mile data for the speedometer. I'm not familiar with the edge, but if you want to find out the correct diameter or circumference to use, find the revolutions per mile data for your tire. This is usually published by the manufacturer.

I just looked up the data for BFG AT in your tire size and got 640. What that means is the tire will revolve 640 times in a mile. Given 63,360 inches in a mile, the effective diameter is 31.512" (63,360/640/3.14159). For circumference in mm you would take 31.512"*3.14159*25.4 = 2,514. It's not far from Jack's figure in this instance.

 

That is why I use radius. It is always right and you don't have to guess a squish factor.

 

You're correct, Windsor, the PCM DOES want revs/mile. For some reason, Edge elected to have the input value be expressed as circumference, which IT then divides into the number of millimeters in a mile to get revs/mile.

It would be just as easy to enter the radius, as i.ride.suzuki suggests, which again can be converted by the Edge to revs/mile through a trivial mathematical computation. What i.ride didn't say (but I knew he meant) was that the radius is measured from the garage floor to the center of your hub, so it includes the "squish".

By the way, the difference between BFG's figure and mine works out to 3 revs/mile, a difference of less than 0.5%. I bet the REAL number is somewhere in between, and it depends on your inflation pressure, loading at the rear axle, and speed.

And, just to bore everyone a bit more, the "loaded" radius of these tires is about 400.2mm, using BFG's figure. If you try to measure this radius, as i.ride reasonably suggests, I suspect you could easily have at least a 2-3mm measurement error. This would give an error in revs of between 0.5-0.75%.

Any way you do it, the value you enter is a "starting" figure. Adjust as necessary.

- Jack

 

Why cant you wrap a string around the tire while the truck is on the ground and then measure the string? Wouldn't this give you an almost dead on reading? Of course the string would have to be ran over, being between the ground and tire.

 

Not really, because the string "sees" mostly unloaded tire. As i.ride. pointed out, it's the radius from the axle hub to the ground that's the governing factor. Take an extreme case: Let all the air out of your tire. Now the "loaded" radius is just a little more than half your rim diameter. But, the rest of the tire is still almost as big around as it once was. This is an extreme case of "squish".

You CAN mark the ground at the very bottom of the tire and drive forward exactly one revolution and measure the distance traveled - it will be close to the "loaded" circumference. Better yet, go forward three revolutions and divide the distance by 3, should help to minimize error.

I actually did the measurement thing, but I found I still had to adjust to get it perfect. I finally found that reducing the circumference by 2.7-3.0% gives a figure that's pretty close.

- Jack