My 2011 mini cooper came with the oem 15" 5-star spooler rims. I don't like the way they look. If I buy 16" rims will there be a drop in gas mileage?

 

No. Motor on.

 

Not due to the 16" rims alone but the wider the rim the wider the tire, the more resistance and grip you may have and the lower the mpg as a result.

So a Stock 15x5.5" rim with OEM 175/65-15 has much less rolling resistance and less grip than a 16x7" aftermarket rim with 205/55-16 or 225/50-16 tire.

These days you can find tires that are specifically engineered for higher mpg but typically they are more oriented to smoother more comfortable ride than toward sharp handling and more grip.

see
http://www.tirerack.com/landing/fuel_efficiency.jsp

Pick a tire that suits your needs then check to see what sizes it comes in that will work with the MINI. You can go with 15x7" rims or 16x7" with about 42mm offset.

 

My 2010 base Cooper dropped 3mpg but the 16" tire was an extreme summer tire that wore out twice as fast as the 15" OEM tire and weighed 21 vs 16 lbs.

 

Tire may weigh more but the wheel might not, depends what you get.

 

The engineer's answer: yes, potentially depending on width, weight, and tire.

The real answer: if you're willing to spend the money for bigger wheels, any change to the fuel cost is pretty negligible. If you're concerned about mileage, driving style will make a far greater impact.

I squeezed a 205 width summer tire around a lighter-weight 16" wheel and dropped about 5 pounds per corner; mileage dropped a bit, but smiles went through the roof. It's a wash.

 

I reckon on a loss of 1mpg per 10 tyre width, but it's hardly scientific since it is so hard to accurately measure small changes in fuel consumption.

So going from the stock Mini 15" size (175/65-15) to the stock 16" size (195/55-16) loses maybe 2 mpg and a 205 width tyre (stock 17" size 205/45-17) loses 1 mpg more.

The small difference in tyre width makes quite a difference to the aerodynamic drag. Weight changes of wheels/tyres are of little importance to fuel economy (though they affect other things plenty).