For me it is less about the cost to go 100 miles than it is about usefulness. We do have a high mpg sedan (Hyundai Elantra ECO - over 40 mpg on the highway), but there are some things it cannot do. Like tow a trailer. We bought a diesel pusher RV in part so that we can stay at my son's place for a couple of nights. Fuel to get there will be expensive, but we won't have a motel bill.