will wider tires decrease mpg

I think it will be a small difference. If you are really worried, look to purchase some low rolling resistance wheels. The only problem here usually is finding one that will fit a 17" wheel.

 

For reference, the MPG hit for the OEM 17" vs. 15" wheels is ~1.25 MPG (per the UK spec sheet).

To me, the 2.5% hit is a rounding error, but others here have varying views.

Whether the tires you're considering will increase rolling resistance depends on which tires you have now, and what the rolling resistance of each is. The extra width and unsprung weight will likely increase rolling resistance, but not necessarily--if the new tires are very efficient compared to the current tires.

 

Check my math:

15": assume 12500 miles a year, @ 50MPG annual average = 250 Gal used
17": 12500 / 48.75MPG = 256 G

Let's round up to 10G of extra gas to be less optimistic.

If you're trying to be environmental then every bit counts. If you're trying to save money, then it's not even worth worrying about. You wouldn't be buying rims if you can't buy another $30 a year of gas.

 

All other factors being equal, wider tires will get worse mileage. Perhaps not a big hit, but they all add up over time.

Tom

 

I have done the math. Go ahead with the wider tires, just be sure to empty your bladder before driving and you shouldn't notice a difference in MPG.