Calculating tire ride height

[mgroeger]

I know this has been asked before and I have tried searching but can't find the answer. This question is regarding tire size calculation for entry into a Tazer for my wife's JL. Yes, I know this is a TJ forum but the fact is there is a lot of solid data over here that isn't always found on the JL forum.
We put D44s w/ 4.10s on her Jeep and needed to buy a Tazer to program that in to the computer. We have "33s" on it from a Rubicon but I know these are not true 33s since they are what I guess is referred to as a metric measurement X/X/R17 vs. 33x10.5.
I want to enter in the most accurate tire size as possible so my shift points are on as well as my mph.
I'm reading all sorts of things...
Measure the spare's circumference and divide by 3.14
Measure from ground to top of tire
Measure from ground to center of hub then multiply by 2

So what is the best and most accurate way?

 

[nk24]

Measure from the ground to the center of the hub and double it.

 

[andy29847]

https://tiresize.com/comparison/

 

[mgroeger]

https://tiresize.com/comparison/

The only problem with that is that it is not the rolling height. As in the tire will always have the same circumference but it's height will be different depending on load of vehicle, tire pressure, etc...
I think it was @mrblaine or @Jerry Bransford that explained how to properly determine your true rolling size to be entered into chip modifiers.

 

[Jerry Bransford]

The only problem with that is that it is not the rolling height. As in the tire will always have the same circumference but it's height will be different depending on load of vehicle, tire pressure, etc...
I think it was @mrblaine or @Jerry Bransford that explained how to properly determine your true rolling size to be entered into chip modifiers.

The rolling circumference doesn't change except as the tire wears. That the height changes as tire pressure changes doesn't change the circumference like would happen with a stretchy balloon.

 

[Steel City 06]

Most brands of tires have a “Revolutions per mile” specification. This is the most accurate way to calibrate a speedometer or estimate engine RPM at a given speed.

For example, Goodyear lists a 33x10.5R17 MT/R as having 643 revolutions per mile.

What brand and model of tire do you have installed?

 

[mgroeger]

The rolling circumference doesn't change except as the tire wears.

Does that mean you would measure the tires circumference (whether loaded or unloaded) and then divide that by PI to get the diameter of the tire and use that number? That was one of the methods I read about but most people were saying to measure the tire with the load of the Jeep on it.

 

[mgroeger]

Most brands of tires have a “Revolutions per mile” specification. This is the most accurate way to calibrate a speedometer or estimate engine RPM at a given speed.

For example, Goodyear lists a 33x10.5R17 MT/R as having 643 revolutions per mile.

What brand and model of tire do you have installed?

That's not what I need. I need to enter into the computer the axle gears (4.10) and the tire size. Based on that the computer will change shift points and adjust the speedometer.

 

[Jerry Bransford]

That's not what I need. I need to enter into the computer the axle gears (4.10) and the tire size. Based on that the computer will change shift points and adjust the speedometer.

Just entering the advertised tire diameter will get it close enough you'll never need to worry about it. Don't overthink it.

 

[Steel City 06]

Choose the closest one for now, and then take it out on the highway and compare the speedometer reading to GPS speed. If it's slightly off, then go up/down a size or two to accommodate

 

[mgroeger]

OK, thanks guys

 

[jjvw]

Measure from the ground to the center of the hub and double it.

Another way is to measure the side of the axle tube from the ground. Where the tape touches the tube is the center of the wheel.

The most accurate way is if you know your axle gear ratio, you can also use GPS speed at a given rpm to determine the true tire size. Adjust the tire diameter in a gear calculator until the the room and speed match.

 

[Vinman]

Just entering the advertised tire diameter will get it close enough you'll never need to worry about it. Don't overthink it.

I agree