Tire size help

[american732]

ok I just put on 38x15.50 18 tires on my truck im doing the math I keep getting 1437 or something like that well the programer dont go that low any help at all thanks John

[ticopowell]

I know it's not too normal, but you can adjust the gear ratios, then set the tire size to something else to get it to work right. I am pretty sure that either in the forum somewhere or maybe even in the pegasus software you could find a calculator for that. Hope that helps.

[Jackpine]

Originally Posted by american732

ok I just put on 38x15.50 18 tires on my truck im doing the math I keep getting 1437 or something like that well the programer dont go that low any help at all thanks John

Your problem is, these are "inch" dimension tires. Your diameter is approximately 38 inches (with no load - really BIG feet!). So, the circumference is pi x D or:

3.141593 x 38 = 119.381 inches.

You have to convert that to millimeters by multiplying by 25.4, which gives an unloaded circumference of 3032.27mm.

But now, your heavy truck flattens the tire at the ground which reduces its effective diameter/circumference. Usually, this reduction is about 3%, so multiply the last figure by 0.97 and you get 2941mm (rounded to the nearest whole number).

Try that last number. If the Gryphon's speed is slower than the real speed, increase it. If the Gryphon's speed is faster than the real speed, decrease it. You dash speedo will always read about 2-3mph faster than the Gryphon, so your goal is to get the Gryphon speed correct (the same as a GPS). When you get the Gryphon's speed right, the dashboard odometer will be correct.

- Jack

[ticopowell]

Yeah, do what he said... he is smarter . I didnt even think to check the numbers, so good luck with that!