If you're worried about MPG and looking at a Challenger, something is wrong. If gas prices and/or MPG concern you, you'd be best with the V6 at minimum or simply going with another car. It kind of defeats the purpose of being so concerned about MPG yet buying a V8 guzzler (R/T or SRT8). If the minor gas cost increase from R/T to SRT8 is going to break the bank and the only thing holding you back from getting an SRT8, money is obviously VERY tight for you and you probably shouldn't be buying either car. There are more important things in life then breaking the bank on a car.

Also, the Viper gets BETTER mileage than the Challenger SRT8. So if you want good mileage grab a Viper

[ 2008 Challenger SRT8 ]
City: 13
Highway: 18
Combined: 15

[ 2008 Dodge Viper Coupe ]
City: 13
Highway: 22
Combined: 16

(Information from http://www.fueleconomy.gov)