Found a small change in tire size messes with engine performance.

[roaniecowpony]

I went from 33s to 35s. I did not notice any issues. But I did later recalibrate just so I could get into heaven.

Im afraid it's gonna take more than a recalibration for me. A lot more.

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[Chemistry76]

Hmmm. Where does all that tire tread circumference go?

You are correct. I have seen another diagram, outside of J-Scan, that showed measuring from the center hub to the ground. That made sense to me then, but now, when I think of squishing the tire nearly flat to the hub, the imaginary extremely deflected tire still has to rotate the same amount per distance.

Should we be measuring the undeflected diameter on a horizontal plane from the hub?

I used from the hub to the ground on my wife's 35" tires, and it worked perfectly. Currently I'm dialing in 37" tires on mine, and I've had to go over the diameter from the ground to the top of the tire.

 

[grimmjeeper]

Hmmm. Where does all that tire tread circumference go?

The tire is rubber and even with steel belts, it flexes.

The tire doesn't remain perfectly round. It compresses when it contacts the ground, affecting the rolling diameter.

 

[cosmokenney]

I'm guessing that I should probably drive a bit before measuring. That way my tires a warmer and the inflation is closer to when driving. ???

 

[Pape]

I did run in that issue. What screwed me up is the original number for the tire are over the real size of the tire from factory. I do think the effect is not only related to size but also weight of the tire.

 

[roaniecowpony]

P

You are correct. I have seen another diagram, outside of J-Scan, that showed measuring from the center hub to the ground. That made sense to me then, but now, when I think of squishing the tire nearly flat to the hub, the imaginary extremely deflected tire still has to rotate the same amount per distance.

Should we be measuring the undeflected diameter on a horizontal plane from the hub?

I used from the hub to the ground on my wife's 35" tires, and it worked perfectly. Currently I'm dialing in 37" tires on mine, and I've had to go over the diameter from the ground to the top of the tire.

It's actually a bit more complicated than just the circumference measurement. When the tire compresses, the tread "squirms". That's why a tire wears while it's rolling straight. But the circumference is a close approximation of the distance it will travel per revolution. Probably a measurement of the circumference near the shoulder is more accurate than at the center. A tire severely underinflated will have a large amount of tread squirm and will travel less per revolution. But, a normally inflated tire will be close to the edge circumference.

 

[2nd 392]

So THAT’S what my problem is

eh, I just drive a diesel so it doesn’t care what size tires I have on it. When my Speedo says 65 I know I’m doing 74… keeps the mileage lower that way too

That’s what I thought too along with raising the speed limiter with a V8. But Nope, after a few weeks I got “Service 4WD- Input shaft speed—wheel speed”. Jeep said recal needed and they couldn’t do it, recommended aftermarket. Gear change also possibly contributing but didn’t effect the speedometer

 

[3TV]

I measured my 40" Toyo MTs when they were mounted on the wheel with correct air pressure before mounting them on the Jeep. and they had a diameter of 39 7/8". After mounting them on the Jeep, with the weight of the Jeep on them they measured 38 1/2" tall. The diameter of a tire is 2x the radius. The height of a tire with a load on it it the radius + static loaded radius. The static loaded radius of my Toyo tires is 1 3/8" less than the radius is because of sidewall flex when it is loaded. So AEV is saying to use 2x the static loaded radius.

Having said all that, I just recalibrated my speedometer using a Tazr and set tire height at 38 1/2". A few days later I drove past one of those 35 mph signs that flash red and tell you your speed by radar. It was flashing 56 mph, and when I looked at the speedometer it said I was going 56 mph. Close enough for me.

 

[Twisted10]

I went from 35's to 37's and lost more fuel but the 392 and 4.56's dont care

 

[Terrymo]

P


It's actually a bit more complicated than just the circumference measurement. When the tire compresses, the tread "squirms". That's why a tire wears while it's rolling straight. But the circumference is a close approximation of the distance it will travel per revolution. Probably a measurement of the circumference near the shoulder is more accurate than at the center. A tire severely underinflated will have a large amount of tread squirm and will travel less per revolution. But, a normally inflated tire will be close to the edge circumference.

Today I learned tire squirm is a real term

 

[3TV]

I went from 35's to 37's and lost more fuel but the 392 and 4.56's dont care

Go to 40s and it still doesn't care.

 

[Twisted10]

Go to 40s and it still doesn't care.

I want to go to 38's and put a 2.5" lift on it. I also want to retire.

 

[YBABRAT]

I am not sure if the small change in tire diameter is enough to change power characteristics from tranny. I am wondering if the value for diameter, being a small change, readjusts shift points in a odd way. IMO transmission shouldn't be that touchy with a variation between 33" tires. I would think the value for tire size should be just for speed and distance calculations.

 

[Camaroboi13]

That’s what I thought too along with raising the speed limiter with a V8. But Nope, after a few weeks I got “Service 4WD- Input shaft speed—wheel speed”. Jeep said recal needed and they couldn’t do it, recommended aftermarket. Gear change also possibly contributing but didn’t effect the speedometer

Aren’t those v8s full time 4wd? I wonder if that’s why you’re getting issues. I’ve been on 37s for 43k miles and my truck still thinks it has 32s. Haven’t had any issues in that aspect.

 

[2nd 392]

Aren’t those v8s full time 4wd? I wonder if that’s why you’re getting issues. I’ve been on 37s for 43k miles and my truck still thinks it has 32s. Haven’t had any issues in that aspect.

Full time, but I don’t see how it would make a difference. Anyway- I broke down and got a Tazer that also allowed me to reduce the TPMS eliminating cold morning low tire warnings and turn off the nannies . But I have the aftermarket recommendation from Jeep in writing now in case they blame it for warranty purposes.

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