Adjusting your tire size in the Custom Options menu on the Gryphon has two (three?) benefits. First, if you enter the right value, (and your gear setting is also right) your speedometer and odometer will be correct (that's two). And, since normal shifting is partly dependent on speed, your normal shift points will be correct (that's three!). With odometer values right, you'll know what your real gas mileage is, if you calculate it the old-fashioned way at fill up.
Since tires are compressed by the weight of the vehicle, the distance they roll in one revolution is somewhat less than their unloaded circumference.
Bill has thoughtfully provided a tire size calculator in Pegasus. It's under Calculators->Gear Ratio Calculator. Simply enter your tire size in the first box like this: 285/75-16 and right away you'll see the rolling circumference (the value you enter in "Tire Size" in the Custom Options in the "Old Circumference" window assuming a 5% squish (which is a good starting point). For your tires, I get a value of 2551 mm. 5% seems a good starting point, but you can go down to the "Squish" window and change that value too to see it's effect with different numbers. I've found my tires have closer to a 7% squish, using the calculator.
What's the effect of not including squish? Oh, maybe 3-4% error in speed, distance and shift points. Not really much in the great scheme of things, but still....
If it were me, I'd adjust the tire size anytime I changed tires.
- Jack
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