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[Jackpine]

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

[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.

[Jackpine]

Originally Posted by DarrenWS6

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.

Darren, since the max pressure on your tires is 65, I would not hesitate to inflate them to 45 or 50.

- Jack

[DarrenWS6]

Originally Posted by JackandJanet

Darren, since the max pressure on your tires is 65, I would not hesitate to inflate them to 45 or 50.

- Jack

The max is actually 44psi. I have General Ameritrac's

[Jackpine]

Originally Posted by DarrenWS6

The max is actually 44psi. I have General Ameritrac's

My mistake! Stay with 40 psi. Overinflating invites a blowout, and, premature treadwear in the middle of the tread.

- Jack

[DarrenWS6]

Originally Posted by JackandJanet

My mistake! Stay with 40 psi. Overinflating invites a blowout, and, premature treadwear in the middle of the tread.

- Jack

Precisely. I was considering dropping it to 35psi but I have a road trip coming up tomorrow, so i'll keep them up at 40 for the highway ride. Riding them at 40 for the past 8 months like I have has taken alot of life out of them, and the one burn out I did last year didn't help lol.

As far as the tire size, I think i'm well set now, I did the same route and I hit 2.9 miles when I parked, and I re-set the OD when I was going 55 so i'll trust it.

[Jackpine]

If you are certain the EXACT distance is 3.0, seeing 2.9 on the odometer says now that your TS is too small, by about 0.1/3.0 x 100 = 3%. So, you may want to add back in about 70 mm making the TS equal to 2480 and see how that works. My guess is though, that the distance may not be precisely 3.0 miles.

It's a bit of trial and error to get it dead right, but it CAN be done. On your road trip, pay attention to the highway mileage markers. Over a longish distance, they are quite accurate. Here in Arizona, I calibrated my speedo by noting what the odometer recorded after 100 miles of interstate driving. I was within 0.1 miles, which is an error of only one-tenth of a percent. Using long distances minimizes error.

Another way to get it "close" is to "pace" someone with an accurate speedometer. Use the readout for speed on the Gryphon (the dash speedo will always be about 2 mph above that). If your pace car is at 70, and you read 71, your tire size is about 1/70 x 100 = 1.4% too big.

I'm guessing that 17 inch rims were standard on your truck and that possibly your current tires are OEM size? If so, what was the Ford recommended pressure for the tires? It should be shown on the VIN sticker on the driver's door and there might be a sticker on the fuel door.

- Jack