Notes on fuel efficient driving
This article, titled King of the Hypermilers, was passed onto me by the Canuck Investor (thank you!).
The article is about a person that uses driving techniques and not hybrid vehicles to realize dramatic increases in fuel economy. While he goes through extreme measures, the point is well taken that most actions that people take on the road serve to lower their fuel economy in significant ways. I also find the comments amusing – a lot of them are very bitter people saying that “you will end up killing people” or to that manner. They are obviously missing the point.
An interesting quote is as follows:
If people could see how much fuel they guzzled while driving, Wayne believes they’d quickly learn to drive more efficiently. “If the EPA would mandate FCDs [Fuel Consumption Displays] in every car, this country would save 20 percent on fuel overnight,” he says. “They’re not expensive for the manufacturers to put in—10 to 20 bucks—and it would save more fuel than all the laws passed in the last 25 years. All from a simple display.”
I remember renting some cars in Los Angeles and a few of them had FCDs inside. It was very interesting to see the effect of speed on fuel consumption. On a Pontiac Grand Prix, going 75 miles per hour on the freeway gave you about 28 miles per gallon. Going 80 miles per hour increased consumption to 25 miles per gallon. Mathematically speaking, the optimal point (in terms of saving time vs. saving fuel) is where the percentage change in speed is equivalent to the percentage change in fuel economy. Even if you don’t have an FCD, you can judge this by your tachometer (RPM gauge) as it is a good proxy for fuel consumption.
For example, in the ridiculously old vehicle that I drive, I can go 100 km/h at 2000 RPM. I can go 110km/h (10% greater) at 2200RPM (10% greater). If I go 120 km/h (9% change), that goes up to 2500RPM (14% change). This means that my optimal point is roughly 110km/h in my vehicle. I typically get 13 kilometers per litre of gas at this speed, which is about 31 miles per gallon. I know that if I drive at 90km/h, that mileage goes up to about 16-17 kilometers per litre or about 39 miles per gallon.
It isn’t worth driving at the lower speed because I value my time more than I value the money I spend that goes into the extra gasoline. Also, driving at 90km/h on the freeway is costing the rest of the public money since it is far below the ambient speed on the road. I don’t buy into the arguments that slowing down will “only take an extra minute of your time” since that extra minute of time is very important – it shouldn’t be other people’s choices to make.
Using the tachometer as a proxy is an interesting concept, but it can only work on some automatic transmissions.
The tachometer is a physical measurement of the rate your engine is cranking….on a manual transmission, it is physically connected to your wheels when in gear, so it doesn’t indicate efficiency at all. It is exactly proportional to the rate your wheels are turning, and is not affected by how hard the engine is working. I.e. If you increase your speed by exactly 10% (without changing gears), your tachometer will always increase exactly 10% also.
On many automatic transmissions I suppose you can use fluctuations in the tachometer readout to determine how hard the engine is working, but it would involve a lot of fuzzy math and the details would vary greatly from car to car. Even changing the transmission fluid in a single car would make a difference. But the idea is still interesting–when the ratio of wheel speed to engine speed begins to drop rapidly, you know that the forces you are pushing against are growing dramatically.
I drive an automatic and only when the car clicks into overdrive (around 70km/h, which is a transition around 2200RPM to about 1500RPM) then the linkage between speed and RPM becomes decoupled. I don’t know if other cars work the same way.
Thanks for a very interesting comment. I wish I knew as much about vehicles as I do about computers.