Thread: Hills=good mpg
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Old 01-03-2013, 12:44 AM   #1 (permalink)
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Join Date: Jan 2013
Location: Oregon, US
Posts: 43

ZoomFreakingZoom - '88 Mazda RX7 Convertible
90 day: 19.02 mpg (US)

Geo - '96 Geo Prizm Base
90 day: 32.42 mpg (US)
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Hills=good mpg

Given the choice of a road over a hill and a flat road, I choose the hill because I believe it gives better gas mileage (and it's more fun of course).

First off, from a theoretical standpoint, if both roads are the same distance, are strait, and have no net elevation change, then fuel usage should be identical. Any extra energy that is used in getting up the hill is recovered when going down the hill; both paths require an equal amount of work to traverse.

In real life this isn't the case, though. Gasoline engines are most efficient (power output per unit energy consumed) at high load, but are held back by friction in the engine. In other words, working an engine hard is good, but you don't want to rev your engine too much due to friction losses.

This to me suggests that the most efficient operating rpm is wherever the engine has the most torque. That would probably be around 4500rpm for my car, and probably closer to 3000 or 4000 for most other cars. At this point in the power curve, the engine is producing the most power per rpm, thus is most efficient in terms of power output per unit energy.

On a flat road, the engine will be in top gear and will be turning (hopefully) no more than 3000 rpm, which will be quite a bit less than the torque peak on most engines. On a hill however, you will be in a lower gear (at least most of the time) and typically be working the engine harder and operating much closer to the torque peak of the engine. This means the engine is getting its work done more efficiently, even though it is working harder.

On the way down is when the benefits will really come because you can just coast in or out of gear, engine on or off. Most modern fuel injected engines will not use any fuel at all when coasting in gear, and will use very little if you just let it idle.

I got to thinking about this after getting 26 mpg on a camping trip over a mountain pass and back. I typically get 24 under similar driving conditions, expect without another person and a (tiny) trunk full of camping gear.

Let me know what you guys think.

1988 rx7 convertible. Streetported, racing beat header and presilencer replacing all 3 catalytic converters. Atkins 6 port sleeves. K&N cone filter intake. 22mpg average, 27mpg max. Spreadsheet (updated regularly):
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