I have a 06 mini cooper with 16000 miles on it. Here in the last month i changed from stock 15" wheels and tires to 17" wheels and tires. Before the change i was getting upper 30 mpg and since the change I'm getting lower 30 mpg. Would changing the rim and tire size show this decrease in mpg? Should i be looking at something else that would cause this? Also with the new wheels and tires should I have alignment done on the mini.

 

You bet it could cause a good drop in mpg. You've most likely put a good amount more rotating mass at each corner. That's mass that the motor has to try to get moving and carry. I don't know if your overall tire circumfrance has changed, but if it's larger than stock, then you've also raised your gearing, which in a Cooper would probably make you use a bit more throttle to get the car moving. Tires can also make a difference. If they're wider or stickier than your last set, they'll have more friction/drag. It all adds up. If you want to go 17" wheels/tires, make sure they're lightweight.

 

all right cool i was hoping it was just the increase in wheel and tires. Glad its not something else thanks for the quick response

 

I think Spooled nailed it dead center.

How much weight you added depends on your particular to-from switch, but average OEM wheel weight plus average non-run-flat OEM tire size weight for 15" versus 17" wheels/tires yielded a gain of 13.8# per corner by my calculations. That would be a total of 55.2# of additional unsprung rotating mass. Such a switch would provide about 1.7" of additional tread width which would contribute a little more to reduced fuel mileage.

The actual average weight gain in such a switch is probably less than I computed because most probably go with very lightweight 17s.

Notes:
Av. wt. of OEM 15" wheels = 14.1#. Range = 12.0 to 15.2#.
Av. wt. of OEM 17" wheels = 22.4#. Range = 20.6 to 25.1#.
Av. wt. of 175/65/15 Tire Rack best sellers = 16.5#. Range = 15 to 18#.
Av. wt. of 205/45/17 Tire Rack best sellers = 22.4#. Range = 19 to 25#.