jacksonpt

help me understand exactly how different tires and wheels affect (effect?) mpg and power. I know the generalization that bigger/wide tires will hurt mpg and power. But I want to take it a step farther.

Does tread pattern have a noticable affect?

I assume tire width does, right?

Obviously tire height does.

Here is why I ask: I'm currently running 33/12.5-15 Firestone Destination MTs on stock 15x7 rims. My gas mileage averages about 16mpg (not great, but not bad). Power is reasonable under normal driving conditions, but when towing it sucks. So I'm kicking around the idea of getting some new rims and tires for daily driving. Probably looking at 17" rims (Tundra, Sequoia, or LC rims perhaps) and 275/70R17 ATs

That would take me from a 12.5" wide MT to an 11.5" wide AT, and from 15" rims to 17" rims. Logically, it seems like the larger rims would help with gas mileage and power... even thought the overall diameter was basically teh same, there would be 2" less of sidewall, so there wouldn't be so much tire flex to "fight".

Does that make any sense?

Any thoughts? Would 17" rims and ATs offer any noticable improvement in MPG and/or power?

MTL_4runner

Tread pattern (like mudders) can have an effect on rolling resistance and thus MPG....how large a difference is tough to say, maybe 0.5 MPG.

The width of the tire really plays no role in mileage at all (not to be confused with the aspect ratio....ratio of H to W). Width plays a much larger part in cornering and road traction.

Tire height makes the largest difference in power and MPG by far, you want to ensure the tire size keeps you at the optimal engine RPM for whatever conditions you are using them. There is also a larger inertia associated with the taller tires which the vehicle must overcome to accelerate or deccelerate. You can think of the larger tires as a moment arm (like a breaker bar) and the longer the bar the more force you can put on the socket only your engine is doing exactly the opposite.....basically like using the socket to rotate the breaker bar....thus the longer the breaker bar the more torque you need on the socket for the same amount of force on the end of the bar (you need equal force to have the same speed or acceleration). So the typical way to cope with the larger tires is to regear which will raise the effective torque applied at the wheels but you will still be losing some usable power since the inertia of the larger tires is greater than smaller tires and you need to overcome that too. Hope I didn't confuse the crap out of anyone but I thought the example might help.

cubuff4runner

Also remember that the weight of the rim can play a large role in your MPG and speed. I'm not an expert, but I would compare overall weights of tires and rims with the two sizes. Ask people who have put steel rims on their vehicles about this issue. As far as the tire flex and MPG I'm not sure. The only difference with less sidewall on tires is the handling, but I may be wrong.

Lt. Dan

I had some of the same questions, You made it very clear!
Thanx MTL_ !!

Flamedx4

Tire size affects your mileage because the engine drops out of its optimum operating range with bigger tires, and further - during accelleration you have to exert greater leverage to turn em which means use more fuel. (see Jamie's lever arm description) We don't have excess power, and our rigs are designed to operate at an optimum speed at cruising speeds. Lowering your rpm takes you off the hp/torque curve 'sweet spot,' and you use More fuel Less effeciently. The same goes for the gear splits between each shift - changing your final ratio with larger tires throws all the shift points off as well. A big v-8 for example, will almost always benefit from lower rpm cruising, because they have an overabundance of power even at the lower rpm. This can make a difference of up to 4 or 5 mpg.

Tire size affects your power because the engine drops out of its optimum operating range with bigger tires, and further - during accelleration you have to exert greater leverage to turn em. (see Jamie's lever arm description) We don't have excess power, and our rigs are designed to operate at an optimum speed at cruising speeds. Lowering your rpm takes you off the hp/torque curve 'sweet spot,' and you use More fuel Less effeciently. The same goes for the gear splits between each shift - changing your final ratio with larger tires throws all the shift points off as well. A big v-8 for example, will almost always benefit from lower rpm cruising, because they have an overabundance of power even at the lower rpm. This can make the difference between barely needing to shift down to 4th to breeze up a hill or having to shift all the way down to 3rd on the same hill (ugh!)

The tread pattern affects rolling resistance, but its not a big factor unless you go with very aggressive tread (as in Swampers.) Maybe .5 mpg between mud tires and ATs, maybe as much as 2mpg for swampers. The sidewall height has almost no effect on rolling resistance by the way, the contruction of the tire carcass itself and the tread face have the largest effect.
The width of the tire does too play a role in affecting your mileage - a fairly major one for large offroad tires. Wider tires on a light vehicle require lower inflation pressures to achieve decent wear, and that raises rolling resistance - a lot. Narrower tires inflated higher roll easier. I ran 285/85s (15x8) at around 34 psi and got great tread life, with 35x12.50s (15x8) I can't run over 25 rear and 30 front and even then the wear is too much in the center - its costing me fuel to run them that low, but any lower and mileage drops drastically. (Plus the handling gets squishy and bouncy.) (Another example - many years ago on a Honda I went from 155/8013s to 205 whatever they were - 50/15s I think, but anyway of the same diameter - I was careful about that - and lost 3 to 4 mpg in the process - no other difference!)

Rims don't affect your mileage directly. A 33x12.50R15 or a 33x12.50R18 are both still 33 nominally. The 18" combo will be substancially heavier though, so this does present a bit more intertia to overcome during off the line accelleration. I still doubt it would affect your mileage, since you spend the most time at speed where this inertia doesn't hurt you. But, rims are heavy, and those large rims so currently popular do affect your braking! There is no logical argument (or any evidence) to support a theory that larger rims/lower sidewalls would improve mileage or power.

So, if you went to ATs on 17" rims, you would probably see no gain in power (perhaps even a tiny loss but I bet you wouldn't feel it,) perhaps a touch better mileage but I'd bet less than 1 mpg, but you'd get better handling and a quieter ride.

Glenn

Anytime you go up in rim size, you are adding unsprung weight. Even going from 15" rim size to 16" and keeping the same overall tire height will increase the weight of the tire/wheel unit substantially.

mike_d

there have been a bunch of good posts in this thread explaining all the differences among switching tire/rim sizes, so i'm not going to repeat any of that. but i do want to comment on on the above statement. it is true that having 2"less of sidewall flex to fight would definatley help your mpg. but if you're doing this so you can have more power for towing, you're never going to notice it. the only way you'll get any significant towing power is to either regear with some shorter gears, or to put (significantly) smaller tires on the truck. any of the little changes you make, will be insignificant when compared to the performance that the trailer takes away.

mike

Flamedx4

Absolutely!