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rics1997 04-06-2012 10:05 AM

Tire weight vs Friction and my new tire first impression
In the battle of weight vs friction (Unsprung weight vs rolling resistance)

Friction (rolling resistance) is the Biggest loser

Well, there is so much information on how weight effects gas mileage and which tire did better in testing. And sure weight obviously does play a factor as shown in the October issue of JP magazine. But they used same size tires or at least close, just compared different weights like the light weight Duratrac vs the heavy weight Nitto. Of course the Duratrac did better in the tests.

Now when I went from stock 255/75/17 Goodyear SRA to the larger 285/70/17 Goodyear MT/r w/kevlar I lost 2 mpg. That tire was both heavier and wider so there was no way to tell which was the biggest factor. The weight or the friction from the wider tire.

But now I have gone to a lighter tire but wider with the 33X12.5X15 Duratrac's, I got to compare a heavy but more narrow 285/70/17 MT/R to the lighter but wider Duratrac. Both are called 33's but the 285 is actually a touch taller in actual measurement.

GY Duratrac 33X12.5X15 weight is 49lbs and 12.5" wide
GY MT/r w/kevlar 285/70/17 weight is 58 lbs and 11.5" wide

With the heavier mt/r 285's I got 15.46 mpg on average
with the lighter Duratrac 33's, I am getting 15.3 mpg average after two hand checks.

So it looks like the wider tire causing increased rolling resistance decreases gas mileage more then the weight of the more narrow heavier tire. Not a huge difference mind you but still the wider tire does play a little more factor in my first tests. I did do a couple hand checks of the 285's prior to changing tires

So if I had to do it again, I may have went with 16" wheels and 285/75/16 over the 33X12.5X15.

And that is for two reasons. Better gas mileage from the more narrow 285's and the fact that the high backspace of the 15's at 3.75" and wider tire is slinging mud all over the Jeep even with the lightest rain.

The 33's look great though

Fishinfool 04-06-2012 06:31 PM

Interesting. Thanks for taking the time to post your results. Are you an engineer by trade?

strider_mt2k 04-06-2012 06:35 PM

Are you experiencing any additional smiles per gallon though?

I mean, those ARE nice wheels.

kjeeper10 04-06-2012 06:48 PM

4351 Attachment(s)

Originally Posted by strider_mt2k
Are you experiencing any additional smiles per gallon though?

I mean, those ARE nice wheels.

This ^

Beastmaster 04-06-2012 07:26 PM

Thanks for the write up. :thumb:

MTH 04-06-2012 07:29 PM

Not sure what to make of it, but when I went from the stock rubi set up to the 33x12.5R15 DuraTracs and 15" alloys, my mileage was essentially unaffected. I was averaging anywhere from about 17.5 to 19.5 mpgs before, and that's about what I'm averaging now. 2010 JKU 6 speed with 3.21 gears. :confused:

Jurhip 04-06-2012 07:48 PM

Please don't take this the wrong way, but since you switched tire make and model each time, I think the results are inconclusive.

Additionally, and I think this is most important, wider tires significantly increase aerodynamic drag. If I had to bet, my guess would be that the increase in aero drag is what caused the drop in fuel efficiency and it is not due to rolling resistance. Again, it is hard to determine because weight and tire design changed at the same time.

rics1997 04-06-2012 08:00 PM

Not make but yes model (all where goodyear) and I am pretty sure that since car companies consider rolling resistance a key factor when dealing with corporate clients buying fleet vehicles, it is more likely the biggest player. I would think a 1 inch friction increase on the road plays more into it then 1" of air drag. And again it wasn't by much. my mileage decrease was only around 1%. Not that much really. Just an observation when going from a heavier tire to a wider one. Of course there can be other factors like tread pattern but again just a first impression.

JeeperJake 04-06-2012 09:26 PM

Thanks 'a ton' for such a nice techno report.

In the interest of data comparison; I'll summarize my own observations;

08JK auto 3.73; bought @ 15000 with stock OEM 255/R70-18 Bridgestone Duelers; over 10,000 miles mixed driving mostly country highways and a little freeway and a little trail: easy to average over 21mpg, highest was highway trip 250 miles @ nearly 26!!!

New tires Kelly KTS 275R65-18. 3 longish road trips over 1500 miles each; 9000 total; same kind of DD country roads mostly; hard to average 20mpg per tank, although sometimes readout AND GPS calculator will report ~20.9 max. Far more tanks average 18~19 mpg with these tires.

New Kellys are far heavier although I haven't found the official weight. They are considerably wider.

My theory was to maintain as near circumference as OEM. These are listed within a couple tenths of inches, at IIRC 32.8".

I notice more time the auto wants to shift out of OD on easy grades pre-switch that was easily held in OD.

Typically around 55 mpg @ ~1700 RPM for both tire sizes.

My opinion currently is for my type of driving, the OEM 2 ply Duelers gave about maximum MPG one could expect from my Jeep configuration.

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