I know is sounds stupid but I was wandering is some one could help me figure out my tire size on my truck, on the EDGE programmer you have to convert the tire size into a number. God know I am not the best mathematician.

I am running 305/55/R20 Nitto Terra Grapplers

Again thanks for the help.

 

305/55/20 = 33.2x12/20 i think that is pretty close

 

I came up with 2650 using their formula. If you allow a 5% squish factor, you would enter 2518. If you allow a 3% squish factor, you would enter 2571. Either of those should put you in the ballpark.

 

I got the first one right, but what is the squish factor mean, is it about air pressure. I am running 50 psi in them right now, they are a 10 ply tire

 

The tire height calculation does not take into account that the tire is not that tall when you get in on the ground with weight on it. There are several factors. Amount of air, how flexible the side wall is, weight of vehicle, etc. So, you have to deduct a little for how much the tire squats. Squish factor. Based on yours having 50 lbs and being 10 ply, I would go with the 3% as a starting point. You may not even have that much. My tires were about 5%.

 

Yes, But how much squish do you get running at 70 MPH on the highway?

 

You still get some, since the rotational speed of the tires is not ALL that high, (maybe about 650 rpm) but it will be reduced of course. An important thing to remember is that the wheels themselves do not "squish", only the rubber tires do. The percentages Bluejay was giving apply to the tread circumference and, since only the outer circumference of the tire changes, (the bead circumference can't) the effect is fairly minimal.

- Jack

 

Your best way to get your tire size is to do a rollout. Mark the tire and the driveway, make one full rotation and measure it, but I'm lazy. I have two sets of tires I run, one 265/70/17 and the other 305/50/20. They are very different in sidwall size obviously but both take a 5% squish factor, or 95% of the formula calculation. I have proved the accuracy of my calculations with a GPS. A GPS is the best way to get it right. When I get my monitor accurate, the truck speedo is 2 mph hour faster at 65 mph.

 

Right. I didn't even try to figure out the number. I just used my GPS and kept lowering the number until the GPS and the Edge MPHs matched.

 

ok, here's another question for you guys from another newbie...i just got my edge programmer today.(this thing is amazing!!) my original tires were 275/55R20 i am now running 285/55R20. Should i try to reset my edge for the difference or not worry about it? thanks guys.

 

Why would you not want to get it correct. Just do the math with the formula, reduce it by 5% and enter your result in the Edge.

 

I'd change it. Your NEW unloaded circumference is about 2580 mm. The old circumference was 2545 mm. (About 2% smaller). Try a value of 2511 mm - which should be close to the "squished" circumference of the new feet.

- Jack