Well you of course have different kinetic energies with the two speeds. Kinetic energy = (1/2)*mass*velocity^2 The vehicle's mass is the same in both cases, so we can ignore that as well as 1/2 since it's a constant. So we have (30)^2 vs (60^2) which is 900 vs 3600 So having 60 mph compared to 30 mph is 4 times the kinetic energy.