Unexpected impact of bigger tires

[rickinAZ]

Two months ago, I switched from 35” Mickey Thompson Baja MTZs to 37” BFG KO2s. The decision was helped by the fact that the unsprung weight was nearly identical—65 lbs versus 66 lbs.

I wasn’t concerned about gearing because the vehicle is driven exclusively on-road, and with a close-ratio eight-speed transmission, the only potential downside would be initial takeoff in first gear. With 442 lb-ft of torque available at 1,400 rpm, that proved to be essentially imperceptible.

I’m a creature of habit and my driving patterns are very consistent, which made it easy to evaluate the impact on fuel economy. Surprisingly, mileage has increased by roughly 10–15%.

????

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[dirtwheeler]

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

 

[andrei]

That means that your baseline was on the edge of its efficiency, and something else had to change in order to improve or worsen. But also be mindful of actual tire size vs what manufacturer claims.

If that is your solid number after a month of fill ups, and no flukes like seasonal fuel quality and weather effects - kickass!

Keep track and report again, solid data towards improvement is always appreciated. Unfortunately there is so much bullshit floating around due to someone's stupidity or arrogance, it gets exhausting to keep running into it.
I remember arguing with this one clown, who regeared from 4.10 to 5.13 on 31's (or 32's) and claiming he gets lower fuel consumption while his engine is screaming at 3000+ rpm, i had to quit internet for a month after that dose of retardation.

 

[JL 2D]

This thread should be fun.

 

[ScotM]

Did you tell the jeeps computer that you went to bigger tires?

 

[PotterJLUR]

The 37in K02s are probably the same size or smaller then the 35in Mickeys. BFG always runs multiple inches smaller then what is on the sidewall.

 

[OminousSkitter]

But also be mindful of actual tire size vs what manufacturer claims.

This is the most likely explanation.

@rickinAZ, did you measure the actual diameter when you programmed the tire size?

 

[rickinAZ]

Did you tell the jeeps computer that you went to bigger tires?

yes

 

[rickinAZ]

The 37in K02s are probably the same size or smaller then the 35in Mickeys. BFG always runs multiple inches smaller then what is on the sidewall.

Their true measurement (by me) is 1.5" taller than the Mickey's.

 

[rickinAZ]

Don't forget that the wild cards here are the abundant low end torque of the Eco (it's much less sensitive to gearing) - and - the fact that there was no increase in rotating mass with the upgrade. I haven't seen that combination on this forum, that's why I'm flummoxed. I was expecting a 10% dip.

And...to be clear this is based on observations over the same well-traveled routes, not a full-on Bill Nye style experiment, but the delta is too large to ignore, and I've run through four tanks of fuel with the same results.

 

[3TV]

The KO2 is a milder tread pattern and has less rolling resistance than the Mikey Thompsons. That is the difference you are seeing.

BFGs being "multiple inches" smaller than advertised is an urban legend. I just got 39" BFG KO2s and they are less than an inch shorter than Toyo MTs in a 40" size. And they are 2 1/4" taller than Coopper STT Pro tires in a 37" size. 40>39>>37

 

[andrei]

The KO2 is a milder tread pattern and has less rolling resistance than the Mikey Thompsons. That is the difference you are seeing.

10-15% difference

BFGs being "multiple inches" smaller than advertised is an urban legend.

Okay guy, fuck years of everyone coming to the same measurements, its about the time someone like you showed up and sprinkled us with facts. Rejoice offroad community!

 

[GearWhore]

Do you drive a lot of highway miles? My highway mileage went up going from stock to much heavier KM3s on beadlocks in the 55-65 mph range. Noticably decreased around town and over ~80. Even heavier Falken 38s finally dropped me all around, but I still average 23-25 mpg with mixed mountain driving (26+ on road trips when I stay under 70 mph), and 22-24 with the ski box in the winter. I was actually surprised my stock Ram 1500 Rebel with same drivetrain gets the same or slightly worse mpg in same conditions on 33s.

Even with all the issues I've had, this is one of the primary reasons I'm so reluctant to get rid of this Jeep.

 

[roaniecowpony]

The effects of aggressive mud tread is pretty big compared to an AT. Especially, a mild AT like the K02.

 

[Jeep Wick]

Don't forget that the wild cards here are the abundant low end torque of the Eco (it's much less sensitive to gearing) - and - the fact that there was no increase in rotating mass with the upgrade. I haven't seen that combination on this forum, that's why I'm flummoxed. I was expecting a 10% dip.

And...to be clear this is based on observations over the same well-traveled routes, not a full-on Bill Nye style experiment, but the delta is too large to ignore, and I've run through four tanks of fuel with the same results.

The simple answer is you are running a slightly lower rpm with the 37's and there is your gas savings. Pretty cool you found a larger tire at the same weight, you found a cheat code.

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