SOLAR TODAY Blog


>>Mastering mileage
June 11, 2008, 5:33 pm
Filed under: Fossil fuel

As gasoline prices inch beyond $4 per gallon nationwide, the mainstream press has taken notice of hypermilers like Wayne Gerdes — obsessives who squeeze fuel for every inch it’s worth. NPR and The New York Times have done pieces this week. These folks boast of getting over 100 miles per gallon on the highway in a Prius, or over 200 in an old Honda Insight.

The techniques? Obviously, the car should be well-tuned, with properly-inflated tires. You want to keep engine revs low and speed under the limit. It helps a lot to kill the engine at stoplights and in traffic gridlock (hybrids do this automatically).

More questionable moves, in terms of safety and legality: rolling stops at stop signs, coasting downhill in neutral and with the engine off, cruising through corners without braking, drafting trucks and buses.

I’d love to get 100 mpg, but I can’t run out and buy a Prius. I got 32 mpg on the last tank in the ’96 Subaru, which is pretty good for a car with all-wheel drive. That tank included a 150-mile round trip mostly on highways and several days of commuting around town. In town I turn the engine off at stop lights (during daylight hours only) and upshift early to keep revs under 2000 rpm, or even 1500. On the highway I’ll admit to coasting in neutral (but with the engine idling). I wouldn’t draft a big truck (truckers hate that) but I do believe in reducing drag due to air resistance. To monitor drag, I’ve installed an airspeed indicator, with the pitot tube protruding through the grill. I feel less guilty about high speed if I know I have a tailwind.

If I were really serious about this I’d put in a manifold pressure gauge, which tells more or less directly how much mass (of fuel-air mixture) you’re pushing into the engine at any moment.

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2 Comments so far
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Great tips! However, is it really a good idea to shut off your engine at a stoplight? What about if you are (perish the thought) running your ac? Doesn’t that adda lot of “pull” on the engine when restarting?

Comment by richard burns

Then and now it amazes us at how much aerodynamics affected fuel economy and how stupidly unaerodynamic the cars were and are year after year. All cars could be aerodynamic, and quieter, and more fuel efficient, but so many of them are aerodynamic atrocities. I’ve been running with airspeed indicators for years to have some awareness of aerodynamic behavior during high speed driving and hypermiling. I use my airspeed indicator as part of calculating speed/drag maximums to help reduce fuel use and maximize range. Also, as you’ve probably noticed, it’s amazing how turbulent highway air (airspeed is almost never stable) is and how far a draft can run behind some vehicles (sometimes up to 100 to 200 feet or more). Also, it’s amazing how forward drafts work too (being inside a higher pressure area in front of a pack of vehicles) and how much headwinds and tailwinds affect engine power and fuel burn rate. I figure in a given level-ground drive in highway traffic I increase MPG by around 15% by watching airspeed and inertia over terrain.

Comment by esmith512




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