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mnmarcus · 05-23-2011, 03:16 PM

My 2 cents...
At around 35 mph (average/typical) wind drag EQUALS rolling resistance. The only way to really lower RR is to lighten your vehicle. ( I think 1/2 the weight = 1/2 the RR). You can lower your wind drag by better aerodynamics or smaller frontal area. 1/2 the cdA = 1/2 the wind drag.

If its equally easy/expensive to get X improvement in aero versus X improvement in rolling resistance do the aero mod, even if you drive 35mph 99% of the time. Because wind drag and RR are EQUAL at 35 mph. 1% of the time wind drag will grow exponentially, + any headwind will hurt mpg much more than the same tailwind would help.

Wind resistance is the bigger efficiency killer. Suppose at 35 mph you'd get 30 mpg. Say you then lightened your vehicle by 1/2, now you get 45 mpg (because your wind resistance remained constant). You could get the exact same fuel savings by instead decreasing your cdA by 1/2 (either decrease frontal area by 1/2 or drag coefficient by 1/2, or some other combination).

Now 1% of the time you drive twice as fast (or drive faster and have a headwind). Normally (no weight change or aero mods) your milage would drop to 12 mpg because when you double your speed you quadruple your wind resistance. (RR is more linear/constant).

Suppose you had lowered your RR by 1/2. Your mpg at 70 mph would be 13.33 mpg. If you had instead lowered your cdA by 1/2 you'd get a more reasonable 20 mpg.

These are all theoretical suppositions though. We all know our milage doesn't drop to 12 mpg (from 30 city) at highway speeds. This is because our cars engines are sized to give us extra horsepower for acceleration and hills and internal combustion engines are also most efficient when operating close to full load. So even though it takes 4 times the horse power to go twice as fast, the engine is operating more efficiently, so mileage doesn't drop as much as we may expect.

If you really want fuel efficiency sell your engine and buy a replacement with 1/4 the HP. I bet your milage would more than quadruple, but it would be a real pain to drive.

05-23-2011, 03:16 PM	#17 (permalink)
mnmarcus EcoModding Apprentice Join Date: Feb 2010 Location: duluth mn Posts: 117 Thanks: 20 Thanked 14 Times in 10 Posts	My 2 cents... At around 35 mph (average/typical) wind drag EQUALS rolling resistance. The only way to really lower RR is to lighten your vehicle. ( I think 1/2 the weight = 1/2 the RR). You can lower your wind drag by better aerodynamics or smaller frontal area. 1/2 the cdA = 1/2 the wind drag. If its equally easy/expensive to get X improvement in aero versus X improvement in rolling resistance do the aero mod, even if you drive 35mph 99% of the time. Because wind drag and RR are EQUAL at 35 mph. 1% of the time wind drag will grow exponentially, + any headwind will hurt mpg much more than the same tailwind would help. Wind resistance is the bigger efficiency killer. Suppose at 35 mph you'd get 30 mpg. Say you then lightened your vehicle by 1/2, now you get 45 mpg (because your wind resistance remained constant). You could get the exact same fuel savings by instead decreasing your cdA by 1/2 (either decrease frontal area by 1/2 or drag coefficient by 1/2, or some other combination). Now 1% of the time you drive twice as fast (or drive faster and have a headwind). Normally (no weight change or aero mods) your milage would drop to 12 mpg because when you double your speed you quadruple your wind resistance. (RR is more linear/constant). Suppose you had lowered your RR by 1/2. Your mpg at 70 mph would be 13.33 mpg. If you had instead lowered your cdA by 1/2 you'd get a more reasonable 20 mpg. These are all theoretical suppositions though. We all know our milage doesn't drop to 12 mpg (from 30 city) at highway speeds. This is because our cars engines are sized to give us extra horsepower for acceleration and hills and internal combustion engines are also most efficient when operating close to full load. So even though it takes 4 times the horse power to go twice as fast, the engine is operating more efficiently, so mileage doesn't drop as much as we may expect. If you really want fuel efficiency sell your engine and buy a replacement with 1/4 the HP. I bet your milage would more than quadruple, but it would be a real pain to drive.