Thread: Hills=good mpg
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Old 01-06-2013, 01:51 AM   #9 (permalink)
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Join Date: Jun 2012
Location: California
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Xterra - '13 Nissan Xterra Pro-4X
Team 2G Xterra
90 day: 18.7 mpg (US)
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Here are my thoughts:

If you think that the reason hills return higher MPG is because your engine is working at it's most efficient operating RPM (4500RPM for you, apparently) then if you're on a flat road why not shift from 5th to 4th or 3rd to get your RPMs to 4500? Would that give you best MPG? Of course not. Therefore THAT is not the reason you get better MPG in the hills. It's the natural P&G opportunities hills provide and, again, has nothing to do with the most efficient operating RPM of your engine.

Unless, of course, I'm wrong!

Originally Posted by 13B_88FC View Post
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.
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