Just wondering if you have noticed the gas efficiency with your wheels upgrade (> 17")
Here is my data...mostly highway driving with 23,000 miles so far

1) with 4 stock wheels - 26 -29 mpg
2) with 4 19" TRD wheels - 23 - 25 mpg
3) with two front stock wheels, two rear 19" TRD wheels - 25 - 29 mpg

 

agreed.. wider/heavier/taller wheels = worse mileage..

same with the xA/b.. i get 4-5mpg better on stock tires..

 

I have a long thread about this and my xB.
I went from 33MPG average to 28MPG average.

It hurt, but it's better then my last cars 18MPG BEST!!

 

I only have 18's. The last fill up only got me 270 miles. Why is it so poor? Right now my tC has 500 miles on it. WTF ?

 

Don't forget that it hurts power too

 

Is it that the wheels are heavier, or that they are larger in diameter? If the decrease in fuel economy is due to its weight alone, then you could always look for a larger rim size that is also lightweight. I have seen 18 and 19" rims for sale that weight the same if not less than the stock tC rims.

 

this is the common misconception about rotational inertia (a wheel's resistance to a change of speed). Weight is crucial, but it isn't the whole equation.

The rotational inertia is a function of both speed and radius of the weight from the axis of rotation.

If you were to put on ice skates, and start spinning with your arms out wide, and you pulled your arms in, you would accelerate!!!! Yes, it's true, I've done the experiment on a rotater thingamagic for a physics class. Ice skaters do it all the time.

So, even if you have identical wheel weights, if the radius of the weight is farther away (larger wheel), it will take more power to spin it (hp and mpg). Even if you have a lighter wheel, you CANNOT outperform a wheel of an equal weight of a smaller diameter. And technically, a lighter wheel that is larger than stock may not outperform the stock wheel.

The main benefits of plus sizing, are increased traction(of a wider wheel) and better handling. But you will almost always lose power and mpg. . .

 

^^^ this explains why my setup#3 is close to original mpg with increased traction and better handling at REAR

3) with two front stock wheels, two rear 19" TRD wheels - 25 - 29 mpg

 

Yup, weight + diameter are the main factors. A similar experiment is if you have a very long stick, you can try picking it up at the very end of it, and it will seem a lot heavier than if you would pick it up in the middle. Though this is not rotational, it is a similar concept of distance (radius) and force required to move an object.

 

Looks like the TRD 19s with tires are actually larger in diameter than the stockers. If this is the case (and you can use a tire size calculator to confirm) then your speedometer and odometer will be off. So there's a good chance that your 23-25mpg calculation with the 4 TRDs is actually closer to the 26-29mpg. When you drive on the larger tires, your odometer reads lower than the actual distance travelled, so if it read, say 280 miles you might have really travelled 300. Then when you calculate mileage it comes out too low.

A few pounds a wheel won't make as much of a difference in gas mileage as you're seeing, so my solution sounds more plausable.

 

JasonH, you may be right...here is the tire calculator results...i'm putting back my two front 19" wheels

 

yeah. . . although one is linear, and one is rotational, lever arms, force, and radius and rotational inertia are all related.

It's the same way your tranny reduces the horizontal load. . . except it is translating a linear lever arm with a rotational one.

 

what if the diameter of the tires are the same, but the rims are just bigger (low profile tires) and the weight of tire and wheel altogether are the same? would that make a difference?

 

it depends on the make of the wheel as well, but the majority of the weight is in the rim, not the tire. You shouldn't really be changing the overall size of the wheel, anyway, as it will throw off the speedometer, odometer, and your gear ratios.

But changing to a larger wheel (and assumed that you will get lower profile tires) will increase the radius of the weight, regardless. Unless you find a light weight wheel that can compensate for the increased size(material) and the radius, you will lose some hp and some mpg, as well as braking distances (usually). However, I'm not totally knocking on getting bigger rims, there are definitely benefits; like increased traction and better handling.

But the lighter the wheel, the better the performance. And in the realm of physics, the smaller the wheel, the better. It's all about finding a balance between wheel size and handling. Some people prefer larger wheels, some don't. But most people don't realize how it works and think wheel weight is the only factor.

 

went to 18's. my mileage stayed pretty much the same aroud 26mpg. ive never gotten above 28mpg which was from my first tank. i drive at 80mph around 4k rpm on the freeway, what type of driving results in 33mpgs?

 

He's got an xB

 

My mileage stayed pretty close when I moved up to my staggered 19s. I get 25-27mpg on average w/5-speed.

 

Has any one stopped to consider friction as a cause of loss of gas milage? Does anyone remember the econoboxes w/really skinny tires? I had a nissan stanza that got 40mpg hwy.

 

Yeah. . . there's always personal claims, but you can look up the formula. You can't argue with physics.

The best bet, regardless, is to go for a light weight, high quality wheel. . .

and yeah, wider tires means more friction which means less MPG. But in general, most people who mod are not concerned with MPG. And if you pick your parts well, you will only hurt it by a slight amount.

 

weight. metal weighs more than rubber. hello.