The mpg difference between 17" and 15" tires
 

While this isn't really news to most of you I thought it relevant for newbies. :)

Over past 6 years I have been swapping back and forth between 15" and 17" wheels. I've had a variety of tires on each wheel yet no matter which tire I, choose I observe an instant 4-6mpg hit when I put the 17s on. Lately I have observed an even larger hit because it is easier to employ hypermiling techniques with the 15s because they roll so much easier.

My current setup

15x7" oem alloy wheels (14-15lbs) with Yokohama dB Super E-Spec 185/65/15 tires

17x7" Centerline RPM wheels (15lbs) with Michelin Primacy MXM4 215/45/17 tires

The 17" tires weigh more so the total difference in weight is about 6 lbs more per wheel.

Both tires are rated as low rolling resistance but the extra width of the 17s and possibly tire compound/construction makes them not roll as easily as the 15" setup. Using the same hypermiling techniques in the same parts of my commute will net 50mpg with the 17s and 57mpg-60mpg with the 15s. It is important to note that my commute consists of 98% freeway driving at 60mph with an elevation gain of 1,200 ft. and a total of 96 miles.

In summary the smaller and lighter 15" tires make a pretty huge difference when compared to the wider and heavier 17" tires despite them being LRR. Now consider what this means for high mpg cars that come factory equipped with 17" or 18" tires like the Lexus CT200h or the 2012 Camry hybrid which are both EPA rated at approx 42mpg combined and are rated with the 17" tires. If one were to swap out the 17s for 15s their mpg could potentially rise to 50+mpg! Thankfully I have a friend with a CT200h who is going to perform the swap to see if this idea pans out. :)

My guess is that the difference comes from a number of factors, most principally the weight difference. The wider tire patch, the differences in composition of the LRR tires, and the differences in drag characteristics of the wheels might all affect the outcome of mileage tests.

To get a better assessment of the potential gains, you should look to get the same tires with similar rim designs in different sizes.

While both are rated as LRR, they are apparently not the same.
(make + model + rubber compound, yet even then there are differences)

The wider tyres cause more aero drag.

Heavier wheels+tyres cost you when accelerating - even if you get some of that back when coasting.

The circumference is 0.6% bigger on the 17" - so it's slightly under-registering the miles driven - you might want to compensate for that, but it's not enough to cause a 4-6mpg difference.

Being that the rims are so close in weight, if you put the same tires on both, in the same overall size (same width and diameter, just different sidewall heights), the 17s should actually get better mpg around town, and the same on the highway, as rolling resistance would be equal, and the tires for the 17" rims would then weigh less, bringing the overall package weight down.

Your 4-6 mpg loss with 17s doesn't seem likely. I see less difference with 12 lb. heavier non-aerodynamic tires/wheels

My first post.

I agree the weight will have very little effect on the mileage, especially highway mileage. Weight in general has very little effect on mileage when a constant speed is maintained. If I'm thinking about it correctly (thought experimentation), weight will only have an effect on gas mileage at a constant velocity by increasing the rolling resistance. Therefore, shaving 50 lbs on the wheel+tire assemblies (that's a huge shave) will have an effectively immeasurable effect on constant-velocity fuel economy.

The factors effecting rolling resistance are not just the width, tread pattern and rubber compound, but also the age and condition of the tire (rubber hardens over time = more mpg), and remaining tread depth (less tread depth = more mpg).

Going from an old tire to a new tire of the exact same brand, model, size, etc, can measurably decrease fuel economy. This often leads people to the inaccurate conclusion that the new type of tires they've chosen are significantly less efficient than their old ones.

Furthermore, differences in sidewall height can change the tire pressure needed for the ideal contact patch. If both the 15" and 17" setups are running the same pressure, this can actually be an inconsistency in the testing.

In short, weight is not the reason for the change, and there are too many other unknown variables to make a solid conclusion about economy based on the information provided.

Smaller tires helped my mpg as well. I recently went from 17" to stock 15" and my avg mpg went from 18.8 to 19.3 (I do mostly city driving).

If your tires are sized properly, you'll have nearly the same diameter tire on the different wheel sizes. This means the tires are not smaller on 15" wheels, they're actually "larger", as in there's more rubber for the same diameter. If they are not sized properly, and your tires actually are smaller in diameter, then your odometer will be wrong, displaying more miles than you're actually going. This would deliver an illusion of increased mpg.

Edit: If your new size is the stock size, I suspect the old size was too big, causing the odometer to display fewer miles than actual, delivering an illusion of decreased mpg. Now you're just back to normal, reading the correct mileage.

I tested the accuracy of the odometer by seeing if 1 mile on my odometer was equal to 1 mile maker on the highway, and it was accurate. Did this on a long trip and didn't get off one bit. Also doubled checked the distance on google maps, the trip length matched what I had on my trip odometer. It was also calibrated when I got my knew tires, and so far it's right on still.

I have my old tires, and when I put them next to my new ones the 17"ers are definitely bigger.

Looking at the 2 tires in question - the 15" is an H rated tire and with an 88 Load Index. The 17" is either a V or W rated tire with an 87 Load Index.

So not only is the 17" ever so slightly smaller in load carrying capacity - which would adversely affect RR, , it is likely that the 17" has higher RR due to the things needed to make the speed rating - like cap plies.