I upgrade from the 15" standard wheel/tire to a 16" Rota RB with General Altimax 205/50 R 16 tires.

Unfortunately my MPG took a 15% hit after the transition.

Has anyone tried Low Rolling Resistant tires with good results?

Any other ideas?

 

The wider tire is raising your drag ce by increasing the frontal area. Multiply the difference in tire width (30mm) by the exposed height of the tire (ground to airdam), then multiply that by 2. This is how much frontal area you just added to the car. Probably not the full reason for the mileage drop, but something that remains constant regardless of what kind of dimensionally same tire you use.

Also, while the old and new tires are theoretically the same diameter, if the old were worn badly, you have a difference in diameter here, as well. The new (taller) tires would turn less to travel the same distance. The odometer doesn't know this. So if you are using the odometer, and not GPS, to calculate your mileage, go measure your 15s and compare to the new tires. The taller tires are not necessarily working against you in efficiency. Typically it works for you. But since your miles traveled will differ on the odometer, it throws off your comparison. Again, this is probably a very tiny discrepancy, but every little bit adds up.

 

More likely winter gas mix.you get less MPG. With it.

 

Timeframe wasn't specified, but that's very true.

 

I made this change 18 months ago, and the MPG loss is consistent through the seasons.

 

How much weight difference is there between the 15" and 16" wheels/tires? Rotating weight is multiplied, IIRC.

 

The base 15" wheel weighs about 12 lbs. The Rota RB weighs about 16.5 lbs.

 

I would guess (just a guess) it is something else other than the wheels and tires.
A 15% change in MPG is rather large for the small change you made.

 

How many miles are on your car? I would think that a good fuel system cleaning could help you gain a bit more MPG back. I put a bottle of Techron in the last time I filled up and honestly saw a bit of in improvement. I have thought about replacing the fuel filter as well, but I have heard both goods and bads on this.

Heavier wheels and tires will effect MPG, as will tire width, driving habits, etc... Tons of variables to gauge here

 

As an example I went from R-98's with 205/45/17 runflats to an Enkie RPF-01
with 205/50/16 non-runflats. Weight savings per wheel: 9.5 lbs.

Change in fuel economy: close to a 10% inprovement in town but not really noticable on road trips.

Hope this helps...

 

Don't forget to focus on tire pressures. Running a little higher pressure could reduce resistance and improve mileage.

 

I have the same size wheels and tires which I switch back and forth. Over 10,000 miles the 16's got 10% less mpg while seeing more summer duty. The 16" wheel and tire weighs 3.2 lbs more than the 15's.

 

I believe wind resistance will make more of an impact than weight, especially if the miles are primarily highway miles.

 

What a timely thread. I am debating the better looking Rota RB at 205/50/16 over the stock R100 Spoolers on 205/55/15.
Are you hand calculating or going by computer? Hand calculating is more accurate, especially if you have a running average of the last 5 fill ups.

‘Mini Coop’ has widened his tire from 175/65/15 to 205/55/15 on stock Spoolers and Kumho Ecsta tires, and claims to not have degraded MPG.

Stock Michelin’s on R100 = @ 31.7lbs per/wheel
205/55/15 Kumho’s on R100 = @ 32.7lbs per/wheel
Rota RB 16s with General’s = @ 35.25lbs per/wheel

Minimal effect by wind resistance, likely tire pressure and slight increase in weight. General’s can be inflated to 51psi. Try increasing the pressured to 45psi (assuming your closer to 35psi) and please report back to see if that helped.

 

This has everything to do with your decrease in fuel econ; your unsprung weight has increased by over 37%.
And that's not even including the increased difference on what the 16" tire weighs.

Remember your middle-school Physics class, and specifically Newton's laws of motion...
Well, that heavier wheel combination requires more force to accelerate and decelerate which causes the reduction of fuel econ.

- Erik

 

I have doubts about this as the section of tire that close to the pavement is barely moving forward.

It's a fact that the tire is always still in space at the point of contact with the road and the top of the tire is moving twice as fast as the rest of the car.

So the section of tire from the air dam down is not moving nearly as fast as the rest of the car and so would have its own set of equations for drag coefficiency.

Also, do we even know if any head-on air is reaching the front tires? Perhaps it's getting diverted by the nose of the car which is the purpose of a front air dam; to keep air from going under the car and causing lift.