I used to average 31 mpg when my Cooper had the stock 15" rims and 175 rubber. Now with 16" X 7" and 205 I'm averaging 29 mpg. These averages are based on 5000 miles of driving to and from work. My driving style has not changed. Can I attribute the drop in mileage to the wheels and tires, or could it be something else? It's not a dirty air filter, as I replace those every 10000 miles. Does a 2 mpg drop seem about right?

 

Has your local gas pumps change formulas, like from a summer to winter formula or vice-versa? Your tire swap could have been close to that tire/wheel swap you did. I know when I lived in the NW, and they changed over the gas fomula, my fuel mileage went down by about 3 or 4 MPG. Happened every year while I was there.

 

I know I definitely saw a 1-2 drop in MPG when I went from my 195 to 205, so SURELY this plays a role in dropping your mileage too, especially because you started from 175.

 

I think you are correct in estimating a change in mpg with tires. The way to test it is to change back to the stock 15" rims and see if the mpg goes back up to 31.

Things that affect lower mpg by 2-
Driving in more stop and go traffic
Driving at a higher rpm like 4000 instead of 2500.
Driving with the AC on more or windows or sunroof down more
Not having correct tire pressure
Adding any weight to the car like one or two more people on a regular basis
Changing sources of gasoline and the composition might be different
Driving on loose surfaces
Taking your MINI to driving school, autocross or doing track events.

One thing to note is that the 15" stock rims and tires are very light and narrow with not as sticky rubber compound. Each wheel can weigh as little as 26 pounds (12 for the 7 hole rim and 14 for the 175/65-15 tire). The 8 spoke rim is 3 pounds more.

Now check to see how much your 16x7 rim weighs as well as the 205 tire and I bet you have increased wheel weight at least a little. I'd guess at least 16 pounds for the rim and 18 to 20 pounds for the tire so you might have gained 10 pounds per wheel. This added unsprung weight doesn't help mpg either. You get more grip in cornering but the 15" stock wheels are better for commuting. It's even worse with 17x7" wheels.

 

I agree with Minihune - weight is the culprit.

Alex

 

>Driving with the AC on more or windows or sunroof down morethis makes a difference too..but not so much to reduce 2mpg all the time. I think
minihune's evaluation is sound.

 

Thanks for the responses everybody.

I added about 8-9 pounds per wheel/tire. I had the 8-spokes (15) and went to the ASA AR1 which weighs a little more than a pound more, but the Yoko AVS ES-100 weighs about 7 pounds more than the stock Conti. The 205 tread probably adds a bit of rolling resistance as well.

I'm not in any hurry to put those stock wheels and rubber back on.

 

In my experience wheel weight is not going to effect mileage very much - unsprung weight will change the feel and smoothness of the ride, but I doubt that weight is a factor. Rolling resistance is usually much more of a factor, so you did the correct thing to up the air pressure. Obviously the biggest factor is tire/wheel diameter, since this changes the effective gear ratio. A smaller tire travels less distance per each revolution of the drive shaft. The should produce faster acceleration, but lower mileage. "Lowering" the car with wheels always decreases gas milage this way.

 

For acceleration purposes, wheel weight plays a role in affecting ones mileage. The engine uses slightly more power to rotate those now-heavier wheels.

 

I agree that changing weight is probably the biggest factor, but..........
Since changing the wheel/tire size changes the spedometer reading accuracy, wouldn't it also change the odometer. Using the calculator here: http://www.miata.net/garage/tirecalc.html I show that now when your spedo reads 60mph, you are actually traveling at roughly 68.1 mph. If you are doing your mpg calculations based on the miles between fill up shown on the odometer you would show you have gone fewer miles than you actually travelled, thus giving you less mpg. Am I all messed up here?

 

I was checking the Tire Rack website for diameters of the tires. I found that the diameter of the 205/50/16 Yokohama's is 24.1 inches and the diameter of the 175/65/15 Continental's is around 24 inches also, so there is not much difference there to effect the mileage much.:smile:

 

Thanks for your response.

I went to your link and put in my numbers (175/65/15 vs. 205/50/16) and came up with a 0.1" greater diameter over stock, a circumference of 0.3" greater, 4 revolutions per mile less and a total of only 0.5 % difference.

I do do my calculations based on my odometer readings, but my aftermarket wheels are closer to the stock diameter 15" wheels than the stock 16" wheels(195/55/16)!

 

Thanks jazmini, I was posting at the same time as you.

 

The added wt isn't the only factor. Don't forget the wider the tire contact with the road the greater the resistance/friction.

Example, how much energy does it take to travel 7mph with in-line skates compared with traditional (my age is showing) 4 wide roller skakes???....(and it's not because the better bearings of the in-line). It's because of the larger contact patch. Another example; skinny racing bike tire compared to beach cruiser tire.

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You better believe it! lol

I put some new shoes on the shop truck; 02 F150 not long ago. Some nice MB wheels and Falcon tires. Closest to 'stock size replacement' as they came.

Well.....after one tow trip to CO for a hillclimb race off they came! Back to stock 16s. The wheels and tires combine for a total of 70lbs each compared to the stock ones at 45. You think 25lbs isn't much? Think again.

Not only was the weight up, the mileage down but the truck labored to pull hills it had (and later confirmed to do again) with the cruise on. No OD nearly all the way back over I-40 West was all I needed to know.

Weight is a much bigger issue than some care to think.

Keep that in mind too when shopping brakes....rotational mass, inertia, gyro mass...it all comes down to compromises.