I just ran some numbers to see how the M6 compares and it breaks down to this...
2011 ZR1 - 14 city/20 hwy mpg
2011 Z06 - 15 cty/24 hwy mpg
2011 Base Coupe - 16 city/26 hwy mpg
The M6 says it gets 9.9 liter per 100k on average which equates to about 23.75 miles per gallon
If those numbers hold up, the M6 will possibly be better than the base Corvette Coupe for fuel economy. Seems pretty god to me.
2007 Montego Blue 335i Coupe, Sports Package, Comfort Access!