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Old 02-17-2009, 02:17 PM   #1 (permalink)
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Wrong numbers

I was surfing Polish web pages to see what they have to say about saving fuel and it's mostly "n steps to increasing your FE". In one 12-step plan, step #10 advises to reduce your speed. OK, we all know that, but here comes the fun part:
You will increase your FE by 15% if instead of driving 100 km/h you go 85 km/h."
I know enough about aerodynamics (but please correct me if I screw something up) to tell that a 15% reduction in speed reduces the drag by more than 15%. Drag is proportional to the square of velocity, so if 100 km/h is 17.65% faster than 85 km/h (100=1.1765x85), then the drag @ 100km/h is over 38.4% greater than at 85 km/h. In other words, the drag @ 85 km/h is about 72.24% of what it is @ 100 km/h. So drag decreases by 27.75%. Add to that losses from other sources like rolling resistance and you're saving a lot.

If you are telling someone to slow down to save fuel, 30% is a better argument than 15%. Not everyone liked math at school

e·co·mod·ding: the art of turning vehicles into what they should be

What matters is where you're going, not how fast.

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Piwoslaw's Peugeot 307sw modding thread

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Old 02-17-2009, 02:34 PM   #2 (permalink)
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Certainly a simplification for simple people. On a highway trip I have lowered my consumption by 33% by lowering my speed by 11%.

Aero isn't the only force. Rolling resistance is also important. If we are minimizing RR, simple people will keep their tires underinflated. The eco-driving advises generally says to keep the tires at door placard.

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Old 02-17-2009, 03:19 PM   #3 (permalink)
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"- the drag @ 85 km/h is about 72.24% of what it is @ 100 km/h. So drag decreases by 27.75%. Add to that losses from other sources like rolling resistance and you're saving a lot."

Both wind drag and rolling resistance burn gas, but the rolling resistance is linear with speed, so for back-of-envelope estimate, it is somewhat Subtracted from the savings implied by the aero calculations. Also, the engine is usually less efficient at lower speeds. The scan gauge trumps all theories, though.
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Old 02-17-2009, 10:53 PM   #4 (permalink)
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Originally Posted by groar View Post
An expanded version of that spreadsheet is now available as an online tool here at EM:

Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

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