35s? is the hit to mpg from the extra weight or the extra width?

[STW]

curious...

Each 35 adds 11 lbs (5kg). 62 lb 35s vs 51 lbs for stock Rubicon tires.

If you move to 35s from stock 33s on a Rubicon, is the hit to mpg from the extra weight or the extra width?
Same BFG T/A Ko tires. Same stock Rubicon rims.

This question assumes you have adequate gearing for the 35s so tire diameter is not what affects mpg.
But say if that assumption is wrong. (I have 4.10 gears and the 8sp auto with 2.0T motor. 2dr)
This also assumes you have a small lift for 35s either way; only the height change from the tire swap is affecting mpg in this question.

Looks like going from stock 285s to 315 width (11.2" vs 12.4") could be part of what affects mpg.

But 20% more weight per tire seems like it would have the bigger effect?

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[quick66]

Both... more unsprung weight (e.g. bigger tire) and increased contact patch (more rolling resistance)

 

[Philly_]

I swapped wheels/tires to 35’s on my Rubicon with the 2.0T before I lifted it, and I noticed the difference.

Still drives very well (in my opinion), but you can feel the extra weight at start and stop. Gas mileage was virtually unaffected at maybe a .5mpg loss on average city/highway, but that might depend on how you drive.

My mileage took a bigger hit when I added the 2.5” lift a month later. I attribute this to the Jeep sitting more level now, having more space for wind to get caught underneath, and the increased weight of heavy duty Metalcloak control arms and track bars.

 

[TheRaven]

Unsprung weight is the biggest factor, but aero is the next. The single biggest aero issue is air under the vehicle. This is why full-size SUVs have 8 inch air dams - to cut the airflow and create low pressure under the vehicle. When you add larger tires, you raise up your vehicle and increase air pressure underneath. This negatively affects fuel mileage. Drag is also a concern of course but adding 1" in contact patch is relatively minimal in the grand scheme.

 

[AcesandEights]

...

If you move to 35s from stock 33s on a Rubicon, is the hit to mpg from the extra weight or the extra width?
Same BFG T/A Ko tires. Same stock Rubicon rims.
...

Looks like going from stock 285s to 315 width (11.2" vs 12.4") could be part of what affects mpg.

But 20% more weight per tire seems like it would have the bigger effect?

A wider tire doesn't impact the contact patch, which is one reason a wider tire isn't (usually) better off road. A taller tire increases contact patch. It also increases the revolutions per mile (it's essentially the last "gear" in the drivetrain), as well as rolling resistance. In your example, the additional "width" between 285 to 315 doesn't have an impact, all other things being equal. The increased height however does.

A taller, but narrower, tire is almost always better off road; however, most people think by increasing the width they are getting greater flotation (contact patch). You attain a larger contact patch through a taller tire, especially aired down. So, a tall/skinny when fully inflated has more psi (bite), and allows you to increase the contact patch for softer terrain. This is relevant to your question because most people think the extra width is creating a larger contact patch and therefore increasing rolling resistance, specifically due to that (incorrectly assumed) larger contact patch.

By going from 33" to 35" in height, you are affectively changing the gearing by six percent, which could have a six percent impact to mileage, or speedo reading. In addition to the six percent change in final gear ratio (considering the tire to be a large gear at the end of the drivetrain), you also have additional rolling resistance and weight.

Once an object is in motion....so, the heavier tire is harder to get rolling, but wants to keep rolling farther/longer than the lighter tire, with less throttle input.

To answer your question, the weight has an impact, but the height has more of an impact.

 

[quick66]

A wider tire doesn't impact the contact patch

Sorry, but if that was true every drag racer would be running 315mm front tires instead of pizza cutters.

 

[AcesandEights]

Sorry, but if that was true every drag racer would be running 315mm front tires instead of pizza cutters.

Why? Pizza cutters would weigh less and provide as good or better steering and handling. The drag car uses skinny tires because they're going in a straight line (linearly). F1 use wider tires because they are cornering (laterally).

Contact patch is based on weight of the vehicle and tire pressure. If you increase width, you lessen the length. You don't increase contact patch by increasing tire width.

It's harder to get that larger (diameter) gear (tire) rolling, not because of it's width, but because of it's height (circumference).

Other thoughts:
https://www.roadandtrack.com/car-cu...ught-you-knew-points-of-contact-tire-patches/


...everything you thought you knew (or read on the Interwebs) about 4x4 and off road tires, before today, is wrong.

 

[quick66]

Why? Pizza cutters would weigh less and provide as good or better steering and handling.

Contact patch is based on weight of the vehicle and tire pressure. If you increase width, you lessen the length. You don't increase contact patch by increasing tire width.

It's harder to get that larger (diameter) gear (tire) rolling, not because of it's width, but because of it's height (circumference).

Other thoughts:
https://www.roadandtrack.com/car-cu...ught-you-knew-points-of-contact-tire-patches/

It's true that the reduced weight is a big reason drag racers run skinnies/pizza cutters, but it's also due to the reduced contact patch of the tire. Adding additional air in the front tires (reducing contact patch) will result in ET improvement all other things equal. The increased tire circumference (a.k.a. rollout) will definitely affect gearing and acceleration. Can also positively impact traction in certain situations

BTW, I'm not basing my comments of of anything I've read on the "interwebs." I'm basing my comments off of 20 years of drag racing competitively in NHRA D2, performing hundreds of hours of testing, keeping copious notes in my log books, using a weather station to normalize data variables, etc...

 

[Zandcwhite]

The width and height both negatively effect drag as well, likely the biggest cost in mpg at least at freeway speeds. Rolling resistance is pretty minimal at 75mph thanks to the inertia of the spinning tire, but pushing a taller and wider brick into the wind is going to create even more drag at speed. Everyone acts like they can out think the inevitability that your lifted Jeep on 35’s isn’t going to get the same terrible mileage it did stock, you can’t. Bigger, heavier, less aerodynamic is going to have a cost, especially at speed. Buy a Prius if you want good fuel economy, it just is what it is.

 

[entropy]

My brain doesn't understand how a skinnier tire can have the same contact patch as a tire that is the same height yet wider. Makes no sense.

Maybe the contact patch gained with width is somewhat "negligible" compared to the contact patch gained with height?

 

[lowmpg]

Here is another thread from R&T somewhat contradicting the first link provided.

https://www.roadandtrack.com/car-culture/buying-maintenance/a21776425/wider-tires-more-grip/

But as far as the question of 'how is a wider tire have a smaller contact patch' well here...

 

[CenTX JPR]

A wider tire doesn't impact the contact patch, which is one reason a wider tire isn't (usually) better off road. A taller tire increases contact patch. It also increases the revolutions per mile (it's essentially the last "gear" in the drivetrain), as well as rolling resistance. In your example, the additional "width" between 285 to 315 doesn't have an impact, all other things being equal. The increased height however does.

My brain doesn't understand how a taller tire can increase revs/mile.

 

[STW]

I swapped wheels/tires to 35’s on my Rubicon with the 2.0T before I lifted it, and I noticed the difference.

Still drives very well (in my opinion), but you can feel the extra weight at start and stop. Gas mileage was virtually unaffected at maybe a .5mpg loss on average city/highway, but that might depend on how you drive.

My mileage took a bigger hit when I added the 2.5” lift a month later. I attribute this to the Jeep sitting more level now, having more space for wind to get caught underneath, and the increased weight of heavy duty Metalcloak control arms and track bars.

thanks Philly, what 35s did you switch to? (assuming you started with the stock BFG TAko 285/70)

 

[redracer]

I'll echo what others have said. I currently have 315/75 R17 Mickey Thompson ATZ P3's. They are great, but I want to go narrower due to weight and rolling resistance.

My next tire will probably be a 35x11.5 R17

 

[2nd 392]

My brain doesn't understand how a taller tire can increase revs/mile.

Taller tire reduces revs/mi. , which is why my speed is 3.5 mph faster than indicated on speedometer at 65mph. going from 285/70–35/12.50. Matches the math and GPS verified. MPG had a constant slow decline over ~a month to about a 1.5 mpg decrease. No recal (also 4.56’s swap) would also be a factor.

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