Physicist required.

What is going to hurt MPG more from these two tires. I have googled and searched. But I end up in the JEEP forums. Who knows if they know what they are talking about ?!?

What hurts more, weight or diameter?

BOTH Nitto Terra Graps G2

A larger Diameter tire but that is lighter 43.72lb
275/65 r18 @ 32.09 in

OR

Smaller diameter but heavier with larger road surface.

286/60 r18 @ 31.46 in Diameter @ 49.3 lb

 

You need math. Diameter of tire And gearing need to be mathed up. Otherwise lower weight tire equals less mass equals better mpg

 

It's a combination of both. The mass moment of inertia is what you're after. It's a calculation based in the mass and how far that mass is from the center of rotation. This is really only a factor (theoretically) during acceleration. There are other factors such as friction that will effect fuel economy.

 

If I were to gamble I'd bet the 275/65R18 tires would yield better mpg.

 

I doubt the difference would be measurable unless under ultra controlled conditions. In the real world, many other variables would have more profound effects on mileage. Bottom line: probably negligible.

 

Sweet thanks TW. 275s it is .

 

Sweet.

PS. Since we are talking physics, the surface area touching road will be the same regardless of the width of the tire (assuming you run the same air pressure in both, which isn't based on width so there's no reason you wouldn't run basically the same air pressure in both).

 

For real ? Didnt know that. Appreciate it . Thanks

 

No physicist required. There's the interwebz. It's just search verbiage that yields results.
This site can calculate the math and give you most of what you're asking for and more.

http://hpwizard.com/rotational-inertia.html

 

Thanks .. that's awesome

 

Taller tire, taller final gear ratio, less fuel used at cruising speeds. Potentially more fuel used if a lot of stop and go driving.

Lighter tire, in terms of MPG, always the winner. It also helps with ride quality and several other things affected by unsprung weight.

 

Great . Awesome advice . Thanks

 

Not true across the board a wider tire has more drag on the road.

Too many factors. Tread compound. Pressure at volume, heat, deflection of tire itself when forces act on it.

 

Do you mean more contact friction or more air drag?

All things equal, a wider tire has the same contact patch area as a narrow tire. The contact patch is a function of the pressure in the tire. Also, friction (traction) is independent of contact area (and pressure for that matter in all but extreme cases). A wider tire could potentially have higher air drag than an equivalently tall narrow tire due to the larger frontal area.

 

Load increase on a narrow tire will cause pressure decreases in essence lengthening contact patch (longer or wider contact patch could mean more friction on the road). Deflection of the sidewalls alters these things as well. Two different tire diameters would have different length contact patches even at a given witdth.

Lots of things to factor and each variance has so many things to consider. I'm not saying as a general rule of thumb the lighter tire wouldn't yield positive results just that there are a lot of additional factors.

Ps sorry typing from phone at dinner and rushing through thoughts

 

How would the pressure decrease in the tire due to adding load? The contact patch will lengthen but it will lengthen on a wider tire too. The total area touching the ground will be the same for a narrow tire and a wide tire (at the same internal pressure).

They will have the same length and same width (since you said they are the same width tire) contact patches, because the total amount of rubber touching the road has to be exactly the same if they are at the same pressure. The friction for the same compound of rubber will be the same.

 

This really is getting deep. Thanks guys. What a great site to be a part of!

 

again sorry, load increase = volume decrease = pressure increase. however small those changes may be at the given sizes.

I was talking about contact patch lengths at varying diameters. Where this all gets tricky is side deformation, sidewall height will play into how the contact patch grows/shrinks.

I didnt read the OP part about same tire. I was talking about different tires the whole time.

 

in your example the larger diameter tires better mpg not considering air drag


BUT this puts more truck up into the airstream, so, more drag.


put down the bong and go drive yer truck

 

Not sure why u went there .. but they were valid points.. thanks