Help: Tire calculation for EDGE
 

Ok, 3 attempts and I can not get it correct, so here I am:

My 'AA' confession: I am an F150 driver with an edge programmer, and I can't do simple math (apparently).:help:


I have a set of Pro Comp 35 12.5 17 XTreme AT tires and would like to get the correct number in my 'edge.' The book states (p16) TIRE SIZE: 35 - 2792mm I simply want to verify this.

If you can verify, please let me know if it 'checks out' - Then, can you please walk me thru the math as you would a second grader?? :o

I can't walk you through the math, but I just got my edge over the weekend also. I put what the book said also at 2792mm but my speedo was 5 mph faster than actual speed according to my GPS. So I dropped it down to 2610mm and it is near perfect. Only about 1/2 to 1 mile fast now. Next time I reprogram I was going to drop it to 2600mm exactly. Should be about dead on with my 35x12.50x17 KM2's. Hope this helps you! :thumbsup:

Actually even if you do put the correct size in the factory speedo will read 2 to 3 mph high. but the edge will read the correct speed. and the easiest and most acurate way to find out your tire diameter is to place a chalk or sharpy mark on the ground at the bace of your tire and mark the tire at that mark. then drive your truck one tire revolution forword or reverse untill that mark is on the ground again. then measure the distance from your first mark and the mark on your tire as it sits on the ground and that is the tire diameter. mine measured 103 inches for my ne tires. then multiply that times 25.4 to get the mm diameter. easy math.

Hi there,

First, let me tell you... I KNOW this seems like it's way too complicated, but I can assure you that it's not! (Being a visual learner myself, I even put in an example for you.) ;) You can use the following equation to determine your tire’s circumference:

[(width X ratio X 0.02) + (rim X 25.4)] X 3.1415 = tire circumference

Example: Your tire size is 285 / 75 - 17.

Step 1 Multiply Width, Ratio, and 0.02 (285 X 75 X 0.02)
Step 2 Multiply Rim and 25.4 (17 X 25.4)
Step 3 Add Step #1 and Step #2 (27.5 + 431.8)
Step 4 Multiply Step #3 by 3.1415 (859.3 X 3.1415)
Step 5 Your tire’s circumference in millimeters would be 2699!

Good luck! :wave:

I have found that I have to deduct about 3% for the squish factor to be right on. If I don't I am 2 mph fast on the Edge.

Hi there,

Bill wanted me to let everyone know that there is a link on our website (on our DOWNLOADS page) where you can download a tire size calculator which already includes the "squish factor." Who knew? :lol:

That's great! Thanks.

where is this downloads page?

Fixed, you missed the 4.

The simplest and fairly accurate way is this: copy the quoted text into google and change 16 to the radius of the tire from the ground to the center of the axle of the rear tire.
Copy this:
Google output:
Even those that use Bills way, i would like to know how close this comes to your current settings.

When I used Edge's calculations from the book it was dead on with GPS at 80mph. I also tried Bill's way and it through my speedo off by a good bit.

My Edge calculation was off about 3%. Had to reduce it for the squish factor to match a GPS.