“Be kind, for everyone you meet is fighting a hard battle” - Often attributed to Plato but likely from Ian McLaren (pseudonym of Reverend John Watson)

Sunday, October 15, 2006

Efficient speed

It's been well over a year since I began my experiment to increase gasoline mileage in my Jeep Grand Cherokee Limited. Without any doubt, huge increases can be achieved. At the outset of the experiment, in August, 2005, my average mileage indicator on the display was at 14.9 m.p.g. It is currently at 23.6 m.p.g., a whopping 58.4% increase, and an estimated 31.1% above the EPA estimate for the vehicle (18 m.p.h. combined). I should add that the information on the average mileage indicator is confirmed by an extremely detailed, tank full by tank full spreadsheet. I calculate mileage by tank full, five and ten tank full moving averages, standard deviation, and estimated annual savings in gallons and in dollars.

In an earlier post (More on acceleration) I estimated that about 14% of my savings come from reduced rate of acceleration. It might be wondered where the rest comes from. As loyal readers may recall, the other steps I've taken are to utilize cruise control at 55 m.p.h. on highways and freeways; anticipate stops and slowdowns to enable coasting to stops and speed reductions so as not to waste energy by braking; minimize use of "appliances" (air conditioning, headlights, seat heaters, defroster, etc.); coasting downhill out of gear (the savings here are controversial - some maintain that modern computerized cars are more efficient coasting in gear); filling the tires to 2 p.s.i. above recommended maximum; avoidance of drive through windows; and turning the engine off on long downhills and at long stoplights.

Of these, I think it's very clear, based on both theory and the evidence of the instant mileage indicator, that the main contributor to my increased fuel efficiency comes from my reduced highway speeds. My understanding of the physics involved leads me to conclude that the reason for the dramatic decrease in mileage per gallon at speeds above 55 m.p.h. is that aerodynamic drag increases as the square of speed (as noted previously, others say cube, which would make it even more dominant).

I have done a lot of "googling" using search terms involving fuel efficiency, minimizing fuel consumption, etc. and there are many people on forums and blogs who contend that their vehicles are much more efficient at 70 m.p.h., and even 80 m.p.h. than at 55 m.p.h. As I noted in my original article on acceleration (To floor it or not to floor it) there is general agreement that fuel efficiency (m.p.g.) increases as speed increases up to a point where the aerodynamic drag increase overrides the increase in efficiency from utilizing fuel for motion rather than merely running the engine. A further complication is that the gearing and engine parameters for a particular vehicle may make it utilize fuel to develop power more efficiently at some relatively high engine speed.

So is it possible that the above-mentioned posters are correct? It would imply an extremely low coefficient of drag, combined with an engine and drivetrain combination that would lead to terrible low speed performance. Since I don't have such a vehicle, I can't do any experimentation, but I suspect it's wishful thinking on the part of the drivers of those vehicles in an effort to rationalize their behavior. I'm not a psychologist, so I have no comment on why they would have a need to engage in such rationalization.

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