rule of thumb gas savings 70mph 55mph 40mph

[stillsearching]

Does anyone have rule of thumb for how much the typical fuel usage changes say cruising at 70mph steady state, then at 55mph, then at 40mph, for different sized vehicles? (everything from motorcycles to schoolbusses ideally but just some generalized figures since the level of aero and weight may skew the results too much to give just one figure for everything)

[PaleMelanesian]

Well, aerodynamic drag increases with the square of speed, so:
40 = baseline
55 = 1.375x higher. 1.375x1.375 = 1.89, or 89% more drag
70 = 1.75x. 1.75x1.75 = 3.06. THREE TIMES the air drag, or 62% more than 55.

Actual results vary. Higher speed and drag put more load on the engine, so it operates more efficiently than at low load, so the extra cost isn't as bad as this shows.

[MetroMPG]

We're putting together a decent list of speed vs. fuel consumption charts that may answer the question if you can find a vehicle similar to the one you're wondering about:

http://ecomodder.com/forum/showthrea...you-15182.html