So, if you record your RPM in the same gear at the same speed (measured by GPS), at 15 and 30 psi (altering hub height) the RPM will be different?
Re-gearing for 33's
- Thread starter JML
- Start date
So, if you record your RPM in the same gear at the same speed (measured by GPS), at 15 and 30 psi (altering hub height) the RPM will be different?
You did see where I said hub height 2x works fine, right? All I said is going off of the tire rpm also works for a long time and when the tire is newish can actually be more accurate (based on the dude above who measured his and the number was 98.75” based on my calculated 99” vs his measured hub height x2 being 97.75”), and that it has been accurate on at least on the 10 or so tires I’ve tested it with on multiple vehicles ranging from Jeeps to half ton trucks.
Either way both of those methods are 1000% more accurate than the advertised diameter which is never accurate, at all. That was the only point to me commenting. It was not to bicker about whether or not hub height or advertised rpm was more accurate.
This is what I love about you people 
Made it to post #7 on topic, and then another 38 (so far) of internet penis fencing over 143RPM @70MPH
Made it to post #7 on topic, and then another 38 (so far) of internet penis fencing over 143RPM @70MPH
The tires “rolling” circumference will decrease as the tire crumples due to the weight of the Jeep on the tire. A straight line is a shorter distance than an arc with the same two points of intersection.
Last edited:
More complicated to me than it appeared at first glance (and fun to think about):
My underlying assumption is: for each axle revolution, the jeep moves forward a distance equal to the tire’s circumference.
Initially, I had assumed circumference to be a fixed value. But upon reflection, the contact patch is an ellipse that gets narrower left to right with increase in tire pressure. Therefore the center of the tread must have a greater radius than the sides and so a longer circumference (C=2 R pi). If that is correct, I think a decrease in engine RPM at a given speed with increased tire pressure (and hub height) is not directly do to the hub height change (although a specific relation could likely be described) , but instead the stretch in length of the tread that contacts the road. I should note that the above also assumes that the speed at a set RPM actually does increase with tire pressure as @jjvw asserts.
…or I’m completely missing something else.
Have you not learned anything from the wasted posts suggesting 5.13s…this thread isn’t about helping OP anymore.
Hasn't been since page one.
Its all ding dongs, tape measures, and fuzzy math at this point.
I naively attempted to right the ship in post #32 it was futile
I studied fuzzy math in the university like 20 years ago. How is that applicable to our TJ life?
ohh ohh ohhh, I have the 5 speed and 33s. 4.56 or 4.88 if I live in a moderately mountainous area?
Similar threads
