Originally Posted by
Gresp15C
The first site computes 3.2 kW for a 170 pound cyclist going 65 miles per hour on level ground with no headwind. If I didn't have a really bad cold that turns my brain into tofu, I'd work out how 3.2 kW translates into fuel consumption. You also need to know the heat of combustion of gasoline.
This is an erroneous figure since the aerodynamic drag for a bike and rider is largely due to the rider, not the bare bike. That said, a roof rack alone does decrease gas mileage and a rack with a bike or two has a significantly higher effect. Quantifying this will be an empirical experiment as it will depend on the rack, the bikes and the car they are on.
The heat of combustion of gasoline is about 20,000 BTU/pound or about 123,000 BTU/gallon. However, an auto engine has a thermal efficiency in the 35% range so factor that into your calculation when you do find the correct drag/wattage number.