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Old 04-14-2008, 03:50 PM   #13 (permalink)
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Aerocivic - '92 Honda Civic CX
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Originally Posted by Winston View Post
Yes, I am amazed at the huge impact to mpg. Everyone knows that the aero drag increases exponentially with speed, but I am still shocked at the difference.
Once ultra-low Cd vehicles like the Aptera to become available on the market, then this FE penalty when traveling at normal highway speeds would become a thing of the past. When you drop the Cd low enough, then you push the drag wall (the point where the drag curve starts to curve upwards fast) up to well above normal highway speeds, so travelling at 70mph causes very little drop to your mpg's (KW/mile for an EV). And if your engine gains efficiency as you increase its load, then with the proper sized engine and gearing, the MPG (or KW/mile) can be close to constant over a wide range of speeds as the slight increase in wind drag as you go from 30 to 70mph is compensated for by the increase in engine efficiency as you increase its load.

With an ultra-low Cd vehicle, the only place where the higher speed kills your mileage is when climbing a hill, since the accumulation of potential enegy with altitude is tied to vehicle weight rather than Cd, and in this regard an ultra-low Cd vehicle is just like any other vehicle. To reduce the FE penalties of hill climbing, you need as lightweight a vehicle as possible. So to keep your mileage up, you need to drive with load (DWL) as much as possible on the uphills whenever a low traffic density or low speed limit allows you to safely do it. Experimenting with increasing the amount of DWL along my route has been where I have been getting my greatest increases in FE lately.
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