Well if the speeds we are discussing are from 0 to 45 or so, then I agree. However if we are talking about 75, I disagree. The following is from an old post -
Here are approximate drag power calculations for a Ford Crown Victoria:
Cd = coefficient of drag = 0.39 (from Cars.com) AF = Frontal area = 78.2" x 56.8" = 4,442 sq in = 30.8 sq ft (2.9 sq meters)(not exactly correct, since the car is not a box, but it will do for this comparison) V = vehicle velocity (1 miles per hour = 0.44704 meters per second) DA = density of air = .002377 slugs-cubic foot (1.225 Kg-m3) at sea level
Drag Power = AF*Cd*(V**3)*DA-2 = (2.9 sq meters)* 0.39 * (V**3) * (1.225 Kg-m3) 2 = 0.68 plus 1 * (V**3) Kg*m(2)-sec(3) = 0.68 plus 1(V**3)N*m-s
Speed Drag Power mph m-s nm-s HP 30 13.4 1,664 2.2 40 17.9 3,945 5.3 50 22.4 7,705 10.3 60 26.8 13,315 17.9 70 31.3 21,143 28.4 75 33.5 26,006 34.9 80 35.8 45,741 61.3
Aerodynamic drag is only part of the power consumption. The other part is rolling resistance. The power required to overcome rolling resistance increases approximately linearly with speed. I have no idea what the actual number for a Crown Victoria is, but as a guess, I'll use 115 N. So the power required to over come rolling resistance is:
Speed Rolling Resistance Power mph m-s nm-s HP 30 13.4 1,542 2.1 40 17.9 2,056 2.8 50 22.4 2,570 3.4 60 26.8 3,085 4.1 70 31.3 3,599 4.8 75 33.5 3,856 5.2 80 35.8 4,113 5.5
So combining the two factors -
Power Required to Speed maintain steady speed mph m-s nm-s HP 30 13.4 3,206 4.3 40 17.9 6,001 8.0 50 22.4 10,275 13.8 60 26.8 16,400 22.0 70 31.3 24,742 33.2 75 33.5 29,862 40.0 80 35.8 49,854 66.9
These are not perfect numbers, but the trend is reasonable. In order for a vehicle to get better mileage at 75, than at 50 mph, the engine would need to be over twice as efficient at the higher speed. For any vehicle in reasonably good condition, this just isn't going to be the case. The only way it can be true is if the vehicle is really messed up. I suppose if you had a really low coefficient of drag (Cd), or a tiny frontal area, or a thin atmosphere, you might shift the numbers around some, but I can't see any vehicle in good condition can do better at 75 than 50.
Ed
One NASA paper lists engine efficiency (for an Otto cycle) versus percentage of power output. I extracted the following approximate values from this table:
% Rated Engine Power Efficiency 5% 15% 10% 18% 20% 22% 30% 25% 40% 26% 50% 27% 100% 29%
My theoretical Crown Victoria has 224 hp. At 50 mph, it requires 13.8 hp to move, which is 6% of rated power. This implies that the engine is about 16% efficient and that it is consuming power at a the rate 3,658 BTU-min or 4,380 BTU-mile (which is about 26 mpg). At 75 mph, the power required to move the car is 40.0 h.p. This is about 18% of rated power. This implies an engine efficiency of around 21% and that it is consuming power at the rate of 8,078 BTU-min or 6,463 BTU-mile (18 mpg) an increase of 47%. Since my Mother has a Grand Marquis, and I know that the mileage doesn't drop this drastically I know I need to account for other factors (A-C load, alternator load, maybe the rolling resistance is higher, or the engine efficiency curve is steeper). However, the trend is obvious. For a car to get better mileage at 75 than at 50, it would have a really bad engine efficiency versus load curve, and would be a very inefficient vehicle at any speed.
