By Sonia Buckley | October 15th 2009 04:16 AM | Print | E-mail | Track Comments
    I recently went home to visit my family in Limerick, Ireland. I had the usual slog through New York, JFK on the way to Shannon airport, where I was met by my parents. On the ride home from the airport, I was surprised to discover that my father’s driving style had changed markedly. He was slowing down to a crawl before traffic lights, and leaving huge gaps in front of him in traffic. I was worried.

    It turned out however, to be another of my father’s little ideas. Call it a new hobby. He is turning our 1990 Mercedes-Benz (with one cylinder misfiring) into a hybrid vehicle.

    Well, sort of. Actually, technically not at all, since he hasn’t made any mechanical changes to the vehicle whatsoever. Rather unfortunately, he hasn’t managed to add an electric motor/generator or regenerative braking. He has just taken to driving the car much much more efficiently, and using tricks in his driving techniques similar to the technological tricks used in hybrid cars. Since your gas mileage depends dramatically on how you drive, you can adjust your driving with surprising results for fuel economy.

    On the drive home from the airport, my first surprise was that my Dad doesn’t like to brake anymore, unless strictly necessary. When it looks like it might be necessary to brake in the near future, he simply puts the car in neutral and coasts to a stop. Every time you brake you are simply turning gasoline into unwanted heat, without getting out any work. So it saves a lot of energy (while also possibly annoying the people driving behind you) if you avoid having to brake. Hybrid cars use regenerative braking, which saves some of the energy that is normally lost during braking. But sadly regenerative braking can’t be 100% efficient, and you save the most energy if you never brake at all, and simply allow the natural damping forces that act on your car to eventually slow you to a stop. “Look how much of my momentum that guy made me lose” is a pretty common complaint in my Dad’s car these days. He might get a few funny looks as he creeps along in traffic, refusing to stop and start, but as long as I’ve known him he’s never cared much about funny looks anyway. The moral of the story is that planning your braking schedule (caveat: when safe to do so) ahead of time can save you a surprising amount of gas.

    Another thing that makes a big difference to your fuel efficiency is the speed you drive your car. When you increase your speed from 40 mph to 80 mph, your fuel consumption more than doubles, but so does the distance you have traveled. Most people know that there is an optimum constant speed for the greatest fuel efficiency (it’s usually approximately 55 mph). Figuring out the optimum speed for your specific car is complicated however, and depends on the type of car, driving conditions etc.

    Mathematically, the power required to push your car against damping forces down a road is related to the speed the car is traveling at. It can be described approximately by the equation

    P = av + bv2 + cv3,

    where v is the velocity of the car and a, b and c are constants that depend on various parameters like the rolling resistance of your tires, the brake pads and the aerodynamic drag. In practice the best way to figure this out is to have a car with a fuel efficiency readout on it (unlike my Dad’s 1990 model), and see what speed is best.

    Another tactic that my dad employs is called “pulse and glide” by the experts. Once he reaches a good maximum velocity, he puts the car in neutral and coasts until he is going much much slower (this makes more sense in a manual), and then starts accelerating again. Since his foot isn’t always on the accelerator, he uses less gas overall. It works best if you actually turn your car off, but since most cars have power brakes and power steering, this is rather dangerous.

    My Dad has practiced making his trip to work as economical as possible in his car. But he’s not the only one. It’s common enough that a name has been coined for people with a gas mileage obsession: hypermiler. At this year’s Hybridfest Green Drive Expo, a race for fuel efficiency hosted in Madison, Wisconsin, the winner in the non-hybrid category achieved 65.1 miles per gallon (mpg). Considering that he was driving a 1991 Honda CRX, I think that’s quite impressive. He achieved a whopping 241% of the EPA estimate (the winner achieves the highest percentage of the EPA fuel efficiency estimate). The winner of the Honda insight section managed to achieve an amazing 122.4 mpg.

    Some hypermiler techniques are obvious, but just a little bit of extra effort. Maintaining your vehicle regularly is surprisingly important for fuel efficiency. Making sure that your tires are inflated (or even overinflating them slightly) improves your gas mileage by reducing friction from the tires. Don’t leave the roof rack on! And while keeping the baby’s carseat, the old microwave you’ve been meaning to get rid of, and a pile of old magazines in your trunk may be convenient, it will most definitely also reduce your fuel efficiency. Try using the A/C less often and leaving the windows closed to reduce drag (and wear deodorant). Planning your route to have fewer stoplights and stop signs and less traffic helps your fuel efficiency too. If you drive the same route often, you can optimize it to make it as fuel efficient as possible.

    Other hypermiler techniques are a little less obvious. One that I was surprised to learn of involves riding on the white lines on the sides of the roads. Since there is less friction on the lines, you consequently use less fuel driving on them. Cyclists are well aware of this technique. Paying attention to wind conditions can help, you’ll get more mpg with a tailwind than with a headwind. Another (slightly dangerous) technique is drafting. That is, riding right behind a semitruck. By taking advantage of the draft generated in the truck's wake, wind resistance greatly reduced.

    Cars also get lower gas mileage when cold, as the engines are optimized to higher temperatures, and oil is more viscous and lubricates less well. So if you are going to do several errands in one trip, it makes more sense to go the furthest distance first in order to warm the car up, and then to make several shorter stops on the way back when the car is already warm. Warming up you car before you start however, does not help fuel efficiency, since you are wasting gas while idling.

    There are more advanced techniques than even these, but I’m not suggesting that everyone adapt all of the techniques I’m writing about. Some of them are even a little dangerous. But hypermiling can be a good hobby, and a good way to amuse yourself on the drive to/from work. You don’t need try every trick there is, I just think that people should know that how they drive can affect both the environment and their pockets.