Will there be any performance or mileage loss going to a 17" wheel or an 18" wheel?

 

Yes, but it is negligible. I now use 17x10.5 in ther rear and 17x9 up front. I lost an average of about .1-.2 mpg around town and have yet to check the hwy.

 

Rotational mass (spinning, unsprung weight) is the bane of economy (whether it's building HP faster, improving torque, or fuel mileage). Here are some numbers for reference: http://www.miracerros.com/mustang/ww.htm

After switching from stock 15" rims to Bullitt 17" rims, I installed a 25% UDP and still can't match the mileage I got with the 15" rims/tires. I love the way that the low profile tires improved handling, but you (almost always) give up something to get something.

 

So it seems like with defconfire, nothing really changed, but with mcfaraway it had a big effect. I can live with a little loss, but if it affects my mileage by 2-3 thats too much. My performance is already sh***y, so i'm not too worried about that. Would it just be better for me to go with the 17x9" S style rims from Americanmuscle and call it a day??

 

Well I also didnt start with 15" wheels I started on 17"s and just got way wider 17". Which according to his data is a huge weight difference between 15' and 17'. I really had no idea it was that much. that may cause a drop in economy.

 

i didnt really notice when i swithed......dont let that ? be a deciding factor for wheels

 

depends on the rim with the wieght and size of the tire

i went from 225-55-16 to 245-45-17 and didnt notice much of a differance

note: tires for 18"+ are alot more expensive