Not that any true Viper afficionado cares about gas mileage, but on the highway, in 6th gear, it is not uncommon to see upwards of 20 mpg. En route to VOI-5 in Las Vegas in my 97 GTS, I averaged 22 mpg for the entire trip, with some tanks as high as 26-27 mpg. Obviously driving style will influence mileage greatly. Your 9-13 mpg range sounds about right for average Viper driving, especially if you like to exercise the beast often.