Well, in the case of the Subaru BRZ and the Scion FR-S, maybe not as much as we'd have hoped. Sure, they have smallish 2.0-liter four-cylinders and six-speed manual transmissions, but their hunger for gasoline is just average. Official EPA fuel efficiency numbers are in, and the BRZ and FR-S with manual transmissions net 22 miles per gallon city and 30 highway on premium fuel. Still, that's about the same as the 2013 Hyundai Genesis Coupe with the four-cylinder and manual tranny which gets 21/30 on premium, and it's better than the aging Mazda MX-5 Miata which returns 21/28.
The BRZ and FR-S chug less fuel if you outfit them with the optional automatic transmission. They probably won't be as much fun, but they will return numbers close to 25 city/34 highway.