We don't know the MPG of the 2005 Mustang yet...but we should soon. That extra money saved will make up for any small difference.

 

I crunched some #'s a while back, and the mpg averages for the A4/M6 GTO was exactly the same as that of the A4/M5 2004 GT. The M6 GTO seems to really shine on the highways, while the mustang did a little better in city driving (both A4 and manual) and highway (A4 GT to A4 GTO). It depends on tranny and driving style but In the end, averages were dead on. With the new 3valve and its A5/M5 combo and the new Ls2, the differences is unknown. But given the GT runs on 87 octane and the ls1/ls2 premium, the GT would probably be the cheaper alternative as far as fuel costs.

Personally, i'm not as fuel conscious as most consumers and i think that holds true for most enthusiasts.