Unexpected impact of bigger tires

[Terrymo]

If your tire spec shows revolutions per mile you don't even have to do this calculation. That number is already the actual diameter under load and should be dead on with your actual speed if you convert it to a tire size. For example, my 32.1" KO2s spec 649 revs/mile, which is 31.1" rolling diameter.

Is the spec listing revolutions per mile for an unloaded tire at maximum air pressure? Does that change with a loaded tire at normal air pressure? Mickey Thompson lists 37” MTs at 548 revolutions per mile and 36.8 inches. Pretty sure on the Jeep they measured smaller than that. I’ll have to double check,

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[azwjowner]

Is the spec listing revolutions per mile for an unloaded tire at maximum air pressure? Does that change with a loaded tire at normal air pressure? Mickey Thompson lists 37” MTs at 548 revolutions per mile and 36.8 inches. Pretty sure on the Jeep they measured smaller than that. I’ll have to double check,

No, it's for a loaded tire at normal air pressure. That's the whole point. Mickey Thompson is one of the few tire manufacturers who doesn't provide the revolution per mile spec. It's not on their website. Whoever you got that 548 from just fabricated it to match the 36.8, because they didn't understand.

Let's look at a tire that does provide the spec. Toyo Open Country MT in 37x12.5r17 is 36.8" diameter and 565 revs/mile. https://www.toyotires.com/product/open-country-mt/

5280*12/565/3.14 = 35.7" effective diameter on the Jeep.

Remember too that the effective diameter is double the radius from the center of the wheel to the ground, because the fact that the upper half of the tire is unsquished is not relevant since it never touches the ground in that condition.

You would find if you ran that tire and simply programmed 35.7 as the diameter the speedo would be dead on, no need to measure or adjust.

 

[Terrymo]

No, it's for a loaded tire at normal air pressure. That's the whole point. Mickey Thompson is one of the few tire manufacturers who doesn't provide the revolution per mile spec. It's not on their website. Whoever you got that 548 from just fabricated it to match the 36.8, because they didn't understand.

Let's look at a tire that does provide the spec. Toyo Open Country MT in 37x12.5r17 is 36.8" diameter and 565 revs/mile. https://www.toyotires.com/product/open-country-mt/

5280*12/565/3.14 = 35.7" effective diameter on the Jeep.

Remember too that the effective diameter is double the radius from the center of the wheel to the ground, because the fact that the upper half of the tire is unsquished is not relevant since it never touches the ground in that condition.

You would find if you ran that tire and simply programmed 35.7 as the diameter the speedo would be dead on, no need to measure or adjust.

You’re right the revolutions were from a retailers website

 

[variableknife]

The only logical thing to do is try 39s next.

you spelled 44's wrong.

 

[jadmt]

https://tiresize.com/tires/Mickey-Thompson/Baja-Boss-AT-37X12.50R17.htm

 

[jadmt]

You’re right the revolutions were from a retailers website

https://tiresize.com/tires/Mickey-Thompson/Baja-Boss-37X12.50R17.htm

 

[Ratbert]

No, it's for a loaded tire at normal air pressure.

If I've learned anything from these forums it's that there's no such thing as "normal air pressure".

 

[rickinAZ]

BFG KO2 37-12.5 17 31 lb of pressure

• I placed a 24” level across my tire, at the “noon” position as determined by the center hub. I held the level until the bubble centered to ensure I was properly at the crown of the tire. Using a tape measure and trying to keep it perfectly straight up and down, I measured from the bottom of the level to the floor: 35.5” In a previous post I reported 36.5”. That was a typo.
• To take a different approach, I used a cloth tape measure to measure the circumference of the spare: 288.5cm implying 36.2”. And, yes, I know that there is a difference given the spare doesn’t have the vehicle’s weight

 

[DirtWrangler]

the best measure is measuring from the bottom of your diffs to the cement on a level surface that will really show you how much you gain going 37 to 35 and vise versa...this is at 26psi on the 37's.

The best measure is to get the true mounted, rolling circumference. This is affected by tire diameter, air pressure, sidewall stiffness, etc. If you really want to nerd out on it, mount the tire at your preferred air pressure, mark the side of the tire near the floor, roll the jeep forward, and measure the distance traveled when the mark comes back around. Measuring the diameter of an unmounted tire is an OK approximation. Measuring the difference in height between a mounted and an unmounted tire, not so much.

 

[jadmt]

BFG KO2 37-12.5 17 31 lb of pressure

• I placed a 24” level across my tire, at the “noon” position as determined by the center hub. I held the level until the bubble centered to ensure I was properly at the crown of the tire. Using a tape measure and trying to keep it perfectly straight up and down, I measured from the bottom of the level to the floor: 35.5” In a previous post I reported 36.5”. That was a typo.
• To take a different approach, I used a cloth tape measure to measure the circumference of the spare: 288.5cm implying 36.2”. And, yes, I know that there is a difference given the spare doesn’t have the vehicle’s weight

it was pointed out to me you need a steel tape.... for optimum results must be done and precisely 1:13pm mst with jeep facing nnw in exactly 59°F while the wind is blowing 3knots from the east....pretty simple really.

 

[jadmt]

The best measure is to get the true mounted, rolling circumference. This is affected by tire diameter, air pressure, sidewall stiffness, etc. If you really want to nerd out on it, mount the tire at your preferred air pressure, mark the side of the tire near the floor, roll the jeep forward, and measure the distance traveled when the mark comes back around. Measuring the diameter of an unmounted tire is an OK approximation. Measuring the difference in height between a mounted and an unmounted tire, not so much.

you missed my point..

 

[roaniecowpony]

BFG KO2 37-12.5 17 31 lb of pressure

• I placed a 24” level across my tire, at the “noon” position as determined by the center hub. I held the level until the bubble centered to ensure I was properly at the crown of the tire. Using a tape measure and trying to keep it perfectly straight up and down, I measured from the bottom of the level to the floor: 35.5” In a previous post I reported 36.5”. That was a typo.
• To take a different approach, I used a cloth tape measure to measure the circumference of the spare: 288.5cm implying 36.2”. And, yes, I know that there is a difference given the spare doesn’t have the vehicle’s weight

If all tires were measured this way, we wouldn't know what diameter they are. There would be weight on that tire differences and air pressure differences. Still, it is good data for you to compare other tires you put on your Jeep and you measure with this method.

important aspects of measuring anything is that the method is:
1. consistent/repeatable
2. comparable to other commonly available data measured with the same method

 

[rickinAZ]

it was pointed out to me you need a steel tape.... for optimum results must be done and precisely 1:13pm mst with jeep facing nnw in exactly 59°F while the wind is blowing 3knots from the east....pretty simple really.

Got it. But...we don't observe DST in AZ. How do I correct for that?

 

[jadmt]

Got it. But...we don't observe DST in AZ. How do I correct for that?

pat your head 2X's while blinking 4X's and should be pretty darn close...

 

[roaniecowpony]

Got it. But...we don't observe DST in AZ. How do I correct for that?

Only repeatable way is to drive to a DST time zone.

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