Thread: Wrong numbers
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Old 02-17-2009, 01:17 PM   #1 (permalink)
Piwoslaw
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Location: Warsaw, Poland
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Svietlana II - '13 Peugeot 308SW e-HDI 6sp
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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:
Quote:
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

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