Hypermiling Lesson   Leave a comment

Our road trip from Sacramento to Albuquerque this Christmas was a Hypermiling 102 class for me. I took Hypermiling 101 a year and a half ago when I last did the same trip. For anyone who hasn’t heard about hypermiling, it is a collection of driving techniques and strategies to get the best gas mileage possible. Some of those techniques and strategies are rather dangerous and not entirely (or not at all) legal, but there are other perfectly legal, safe, and sane techniques. Like not flooring it out of the gate. Keeping the tires inflated and the engine maintained. The specific strategies that I’ve started practicing on road trips are anticipation, reduced average speed, and keeping the power output constant, rather than accelerating/decelerating. On flat roads, cruise control is the best way to do this, which is why cruise control is considered by many to be the most fuel efficient way to drive. Cruise control, however, is very inefficient on hills. Since cruise control is all about maintaining a constant speed, the car coasts on the downhill and then accelerates on the uphill, fighting gravity the whole way.

I hadn’t really practiced driving with constant power output before my trip in 2009 since most of my driving is in the city or on flat highways. (Since driving with a constant power output on a flat road means driving with a constant speed, I’m really just talking about non-flat roads here.) On that trip, I left Kingman, AZ on the morning of the second driving day without gassing up. I know what my typical gas mileage and miles per tank is, and I looked at my 3/4 full tank and calculated in my head that I was perfectly fine on gas. Unfortunately, east of Kingman the freeway is extremely hilly. I was merrily barreling down the freeway, singing to whatever CD I had picked to start the day, and enjoying the morning, when I realized that I had burned about half of my gas WAY before the halfway mark to Seligman. After doing the calculations in my head, I started to freak out that I was going to run out of gas in the middle of nowhere. I was mortified. The basic problem was that instead of getting 50 mpg, on those hills at freeway speeds I was getting more like 35 mpg. And that difference leads to a significantly different driving range per tank.

Remembering the articles I had read about hypermiling and the conversations at work about the constant power output strategy (aka constant throttle, but since my husband and I drive hybrids that’s not an accurate term for our cars), I decided that I needed to give it a try. So I dropped my average speed from 75 to 70, but most importantly I allowed my speed to fluctuate and instead kept the pressure on the gas pedal constant. On that trip it was pretty easy to drop my average speed since my choices were drive slower or run out of gas in the desert in the middle of nowhere. That is powerful incentive. My speed generally fluctuated between about 60 and 80 mph as I allowed my car to accelerate with gravity on the downhills and decelerate on the uphills. In order to drive like this, anticipation is a critical skill. The road conditions up ahead, including grade and relative positions/speeds of other cars, have to be accounted for constantly. For example, the amount of acceleration required on a downhill depends on the length and height of the uphill that follows, and also the number of cars on the road. There’s no point in picking up speed only to have to break behind someone going slower at the bottom of the hill. In my opinion common courtesy is also very important: no tailgating, no cutting people off, and no boxing people in if it can be avoided. Interestingly enough, on that section of the highway I soon realized that there were several drivers doing the same thing that I was. That made it fun and easier. And I made it to Seligman with plenty of gas to spare, averaging 40 mpg for the Kingman-Seligman stretch (I gassed up anyway).

On this trip we were driving my husband’s Highlander instead of my Prius. His Highlander is obviously a larger vehicle, and it has a more powerful engine and a power meter on the dashboard. Most people would know that as a tachometer, but in the Highlander it is called a power meter since it is a combined measure of power that includes both the electrical motor and gas engine outputs. I requested the first driving shift on the second day so that I could experience the same hypermiling in a different car, and see if it is easier or harder with the power meter. The first thing that I realized was that the size of the car makes a difference! My Prius was so fun. She flew down the hills and then gracefully coasted up the hills. The Highlander wasn’t quite as much fun on the downhills, and it lost momentum fast on the uphills. To be completely honest, there may have been a mental aspect to my perception since I didn’t have the incentive of running low on gas, and neither did we have the perceived camaraderie of other drivers like us. It also felt more painful to decelerate on the uphills. Unfortunately, America is a nation of power-cravers, and that extends to cars. We build and buy cars with lots of power under the hood and look down on the little car that can’t make it up a steep hill. So being in a nearly new SUV creeping over the top of the hill was a weird sensation. I had to broaden my speed fluctuation to 55 to 85 mph to accommodate the difference between the vehicles.

As for the power meter, it was useful but not necessary, and I could see how it could become a crutch. Driving like this is an exercise in intuition and driving by the feel of the car and the road. Using the power meter draws attention away from that feeling and focusses on just this one little gauge. But it did make it easier to judge power output, obviously. Keeping the same pressure on the accelerator will maintain constant power output, but that can be hard to do on a long trip and the gauge keeps you honest. The gauge also helps to take advantage of gravity. Pressing a little harder on the accelerator on a downhill causes less of an increase in power output than it would on an uphill, but without the gauge it is difficult to judge how much to change the foot pressure. The power meter was crucial for the second half of my Hypermiling 102 class: the Central Valley. On Highway 99, when the traffic was sufficiently light, I was able to keep constant power output by combining cruise control and my hypermiling tricks. Cruise control works on the flats, but it forces the engine to rev up when going over or under a road crossing. So I used the power meter gauge and some trial and error to take the car off cruise control, maintaining the same power output for the dip or rise, and then back on cruise control. Twas a bit challenging, and probably didn’t really make a difference to our gas mileage, but it was personal.

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Posted January 11, 2011 by mayakey in conscious living, energy use, travel

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