[DarrenWS6]

Originally Posted by JackandJanet

Darren, 3.1415 is pi (It's really closer to 3.1416). When you do this calculation: 17 x 25.4 = 431.8, you get the rim diameter in millimeters. Adding in 255 x 70 x 0.02 = 357 gives the additional diameter of the wheel contributed by the tire, so the overall diameter of the wheel is 788.8 mm. Now multiply by pi to get the unloaded circumference, which is about 2478.1 mm. OK, this is how far a wheel will roll in one turn if you take it off the truck and roll it across your garage by hand.

But, the truck's weight "flattens" the tire, effectively reducing the diameter under load. On the truck, one revolution will cover less distance than it did when off the truck. I've found for my tires that about a 3% reduction in circumference is about right. I suspect this will be close for your tires too, so subtract 3% or 74.3 mm from the unloaded circumference of 2478.1 which gives 2403.7 (round that to 2404).

You can avoid all the math, of course, by just entering your tire size in the "Gear Ratio Calculator" that is one of the tools in Pegasus. The "Old Tire Size" you should enter is 255/70-17. The calculator has a default "rubber squish" of 5% which gives a circumference of 2421 mm. The calculator uses higher "squish" values because the rim doesn't deform, only the tire does, so the percentage change is being applied only to the tire's contribution to the diameter. I've found my calculator "squish" needs to be about 7%, and if I use that on your tires, I get 2399. A 6% squish gives 2410. So, you can see that the 3% total reduction I used earlier is between 6 and 7% rubber squish.

Anyway, try 2404 (or maybe 2410, since you inflate your tires higher than mine) as a starting point. I think it will be close. (And I hope I haven't confused you too much)!

By the way, 3.3 recorded miles in 3.0 actual miles is a huge error! (About 10%). And, showing more distance on the odometer than actually traveled says your tire size is too large.

- Jack

Thanks yet again Jack for the information. I will try this again. Hopefully this will be resolved soon. I'll put in the 2410mm size and go back down that 3.0 mile route and see how it goes. I keep my tires at 40psi so that should be a good starting point.