12-07-2017, 07:43 AM
WF Supporting Member
Join Date: Sep 2014
Location: Cheyenne, WY
Using a Tire Size and Conversion Calculator, here are some numbers. Imagine if the rim and the contact patch dropped an inch(less sidewall). In my thought it would reduce the effective circumference. I adjusted the second tire to get a ~1" sidewall difference.
315/75R17 - 315/67R17
Current New Difference
Sidewall Height: 9.3'' 8.31'' -0.99''
Section Width: 12.4'' 12.4'' 0''
Tire Height: 35.6'' 33.62'' -1.98''
Tire Circum: 111.84'' 105.62'' -6.22''
Rev. / Mile: 566.52 599.89 33.37
If your speedometer reads 65mph, you are going about 61.4 mph
So going by this, first guess was a miss. Kinda. If the axle was locked there would be a speed difference. But when open, each individual tire would still be going the same speed as all the others. I am guessing that the wheel "speed" sensors are actually determining speed vs rotation.
It also looks mathematically exponential
If your speedometer reads 5mph, you are going about 4.7 mph
10/9.4, 20/18.9, 30/28.3, 40/38.7, 50/42.7, 60/56.7
I would guess that even if someone forgot to air down a tire for rock crawling and locked axle, it would not be enough to mess up readings. Plus I would bet the class of tires makes a difference. My E load KO2s barely showed any drop when I had a flat and the tire pressure was 12psi.
So anyway, this is just stuff I think of when I am having my bouts fighting insomnia.
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