So what robs the HP? Weight or resistance? Or both? - Jeep Wrangler Forum
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Old 03-02-2016, 05:55 PM
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Question So what robs the HP? Weight or resistance? Or both?

Lets take a 2016 Stock JKU, 3.73 Auto

You toss on BFG KO2, pretty light weight. Say sizes 285/70, 315/70, 34x10.5 and 35x12.5

They are all within few lbs of each other. But is it the weight or tread contacting the ground that brings down the MPG?

If its a choice between 285 or 34s...why the difference

same goes for 315 and 35s

I had a 2012 JKU sport (auto and 3.73)and ran MB72 and KM2 35x12.5x15... It wasn't bad at all...but definitely would slow down real quick when let off the gas.

Would 315 KO2 be better on Economy then 35KM2s.. again with weight only being 3lbs off?

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Old 03-02-2016, 06:15 PM   #2
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The HP is robbed by the diameter. Just to keep the math simple, say you had a 30" tire and changed to a 33" tire. That is a 10% change. Now take the circumference the 30" = 7.8' traveled per revolution and the 33" travels 8.64' per revolution. 10% further traveled per revolution of the tire.

However because the gears weren't changed, your motor crankshaft still turned about 18 times. So with your motor spinning the same, but needing to push the jeep 10% further, the energy has to come from somewhere. So you press the gas peddle down a bit more and MPG suffers.

Now work that backwards if the 33" did not turn the full revolution, but only covered the same distance as the 30". The motor turned 10% less thus generating 10% less hp for the same distance traveled.

There are also other factors including tire to road resistance and decreased aerodynamics that affect HP & MPG.

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Old 03-02-2016, 06:27 PM   #3
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Diameter does not affect the horsepower. You can always go to a lower gear to counter the change. But tread, air pressure, width, will all develop more rolling resistance absorbing more horsepower. And weight affects how fast the tire will spin up, accelerate. A heavy tire will actually maintain momentum and take longer to slow down.... all else equal.

The only magic to fuel economy is light, skinny, high pressure. Relate that to a mountain bike vs a racing road bike. Same with gearing... how often do you shift a bicycle riding thru a mountain valley.
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Old 03-02-2016, 06:45 PM   #4
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Diameter does not affect the horsepower. You can always go to a lower gear to counter the change. But tread, air pressure, width, will all develop more rolling resistance absorbing more horsepower. And weight affects how fast the tire will spin up, accelerate. A heavy tire will actually maintain momentum and take longer to slow down.... all else equal.

The only magic to fuel economy is light, skinny, high pressure. Relate that to a mountain bike vs a racing road bike. Same with gearing... how often do you shift a bicycle riding thru a mountain valley.
Yes lowering the gear ratio will give the HP back. The example given was just changing tires per the OP.
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Old 03-02-2016, 06:49 PM   #5
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Yes lowering the gear ratio will give the HP back. The example given was just changing tires per the OP.
Why regear... just shift.
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Old 03-02-2016, 06:55 PM   #6
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Why regear... just shift.
He has an auto tranny. It can be reprogrammed though. As you know shifting would work for 30"s to 33" change but to a 36" dia would really be pushing it.
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Old 03-02-2016, 07:23 PM
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Lots of times on traveling across Kansas, it was just easier to keep it out of OD, mainly if it was windy.


But back to the tires... Why have a 315/70R17 and a 35x21.5R17

Are they not about same?
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Old 03-02-2016, 07:36 PM   #8
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Yes. I think it is a generational thing. Some like me like using inches because it is what we are used to. While the younger generation might prefer the metric.
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Old 03-02-2016, 07:49 PM   #9
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It all robs mpg's. Bigger tires, heavier tires, more contact patch, the jeep is now taller, more wind resistance. Inappropriate gear ratios for tire size brings the motor out of its peak efficiency. There isnt just "One" factor that hurts the mpg's
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Old 03-02-2016, 07:52 PM   #10
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Lots of times on traveling across Kansas, it was just easier to keep it out of OD, mainly if it was windy.


But back to the tires... Why have a 315/70R17 and a 35x21.5R17

Are they not about same?
Well, there's your problem...it is those 21.5 inch wide tires.
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Old 03-02-2016, 07:54 PM   #11
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I'd say IQ points... its a Jeep! Why are we even discussing hp?! LOL!
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Old 03-02-2016, 08:57 PM
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Well, there's your problem...it is those 21.5 inch wide tires.
You should see the rim
Lol
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Old 03-02-2016, 08:58 PM
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I'd say IQ points... its a Jeep! Why are we even discussing hp?! LOL!
Cause we have to find a happy medium... I think
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Old 03-03-2016, 08:46 AM   #14
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First of all, BFG tires are not lightweight.

Second, unsprung weight is a killer...perhaps even in the literal sense if you ever have to make a panic stop to keep from hitting somebody. Yes, the diameter adds leverage against the driveline which robs power, and the pattern increases rolling resistance which robs power, but don't underestimate weight. Heavy tires kill MPGs, power, and braking. Which is why even 1/2 ton crew cab 4x4 trucks come from the factory with P (passenger) tires on them.

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