post your true tire diameters
11-28-2013, 04:58 AM
#11
JK Jedi
11-28-2013, 10:29 AM
#14
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35x12.50r17 KM2 on 17x8.5 on JKUR with 30psi. 33.5" is what I measured and used with me Procal.
Previous JKUR with 315/70r17 Duratrac on 17x9 with 32psi. 33.5".
BFGoodrich website says that size is 34.8.
Goodyear website says that size is 34.4.
Previous JKUR with 315/70r17 Duratrac on 17x9 with 32psi. 33.5".
BFGoodrich website says that size is 34.8.
Goodyear website says that size is 34.4.
11-28-2013, 10:48 AM
#16
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315-75/R-16. Two door JK. 37psi. Nitto Terra Grapplers.
Procal has 33.75" loaded, and me speedo is still off a small bit (likely smaller). On 16x8 wheels.
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Procal has 33.75" loaded, and me speedo is still off a small bit (likely smaller). On 16x8 wheels.
Sent from my DROID RAZR using Tapatalk
11-28-2013, 11:11 AM
#17
Forum Tech Advisor
I see this come up often. Yes, overall height will be less than specified when weighted. However, for the purpose of fitment, the radius is what matters. Take a 37" that actually measures 37" off a wheel. Throw the weight of a jeep on it and from the center of the hub to the ground, it is less than half of 37". From the hub center to any other point, it will be half 37". The bottom flattens, not the rest of the tire.
If the wheel width exactly matched the tire bead width and the tire psi were exactly right, then radius if an unmounted tire might be the same to the top. However, tires flex and bulge. So, the radius to the top when the tire is bulged on a narrower wheel can exceed the unmounted radius.
11-28-2013, 11:27 AM
#18
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It doesn't work that way for speedometer programming. Diameter will give you a correct speed, not radius.
If the wheel width exactly matched the tire bead width and the tire psi were exactly right, then radius if an unmounted tire might be the same to the top. However, tires flex and bulge. So, the radius to the top when the tire is bulged on a narrower wheel can exceed the unmounted radius.
11-28-2013, 01:17 PM
#19
Forum Tech Advisor
Measuring to the bottom while parked may be different when park than when in motion though correct? Wouldn't centrifugal force cause the center of the tire to expand thus creating a different radius/diameter? May be thinking of this wrong and or maybe change is so minor that it doesn't matter
If you measure to the top of the tire while parked and use that measurement in your ProCal/Superchips/Diablosport, you should be close to exact with your speedometer when verified with a GPS.
11-28-2013, 01:35 PM
#20
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Ok and that makes sense. For some reason I thought someone said to measure from center to bottom and I was thinking that it would be under compression but measuring to top makes sense