Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
The closest thing to an instant calculator to get an estimate would be to enter the vehicle information on the following calculator, you'd need the weight of the vehicle + the people and gear (or at least a close estimate), the frontal area in ft^2 or m^2, the coefficient of rolling friction for the tires (average value around 0.010, estimated engine efficiencies, (22-30% at best for gasoline powered vehicles), the energy density of gasoline, and thats pretty much it really)