An automobile’s engine produces 30 hp while being driven at 50 mph on a level highway. In those circumstances, the engine’s power is used to overcome air drag, rolling resistance between the tires and the road, and friction in the drivetrain. Estimate the vehicle’s fuel economy rating in the units of miles per gallon. Use a typical engine efficiency from Table 7.6 and the density of gasoline in Table 6.1.

Table 6.1

Table 7.6

Density, p Viscosity, u Fluid kg/m? slug/ft kg/(m - s) slug/(ft s) Air 1.20 2.33 x 10-3 1.8 x 10-5 3.8 x 10-7 Helium 0.182 3.53 x 10-4 1.9 x 10-5 4.1 x 10-7 Freshwater 1000 1.94 1.0 x 10-3 2.1 x 10-5 Seawater 1026 1.99 1.2 x 10-3 2.5 x 10-5 Gasoline 680 1.32 2.9 x 10-4 6.1 x 10-6 SAE 30 oil 917 1.78 0.26 5.4 x 10-3