Fuel Efficient Driving--a Learning Curve.

            The new Ford Focus I purchased last month has a feature I’ve never owned before; a little screen on the instrument panel which tells you how many miles per gallon your car is delivering at that moment.  The car has an EPA fuel efficiency rating of 37 mpg on the highway, and 31 in the city.  But they issue a caveat that mileage may vary, depending on the driver, and on other factors.  In practice, I’ve been doing a little better than those numbers.

            I am not a novice driver.  Over the years, I have owned two dozen cars and have driven over a million miles. And I had come to believe that I knew something about driving efficiently. While the new “mpg readout” has delivered only a few surprises, it has exposed innumerable subtleties I would never have expected. But first, the surprises.

1.     Gas efficiency is more dependent on engine temperature than you would imagine. We all know that in the old days, when it was 20 below zero Fahrenheit, and your automatic choke remained at least partly  closed for the first five minutes after the engine started, this used a lot of fuel. And during this first five minutes, the engine ran roughly and delivered very little power, if it could manage to stay running at all. But that’s not what I’m talking about.  On a hot summer day, a modern engine starts instantly, and with a modern computer controlled fuel system, it runs smoothly and delivers full power immediately.  And in about a minute and a half, the engine temperature gauge says that the engine has reached normal operating temperature. You would assume that the mileage delivered at this point would be about as good as you’re likely to get.  But you’d assume wrongly.  On the highway, my car gets only 35 mpg at that point. But if I run it hard for two more minutes, it starts to get over 40 mpg.   My explanation:  Just because the temperature gauge reads normal does not mean that all parts of the engine have gotten as hot as they are going to get.  And the fuel computer probably does not begin to lean out the mixture until the exhaust gas temperature is really hot.

2.     You would imagine that on the highway, maximum mileage would be delivered when operating on “cruise control.”   When these cruise control devices were first sold, back in the 1970s, the promise was that this device would deliver more fuel efficiency, since the cruise control would keep the vehicle at a perfectly uniform speed. But my experience so far has not borne this out.  On a perfectly flat stretch of highway, the cruise control delivers about as good a mileage as I’ve ever gotten.  But on a road that’s even slightly hilly, I can do better driving myself.  I do not try to keep a constant speed; instead I keep a constant throttle position—and I let the car slow down a bit going up each hill, and then let it speed up going down the other side.  And this seems to get much better mileage than trying to maintain a constant speed. But now, the subtleties:

3.     While we would all assume that driving into a headwind should consume more fuel that driving with a tailwind, I would never have guessed how little wind it takes to make a noticeable difference.  With a wind of less than ten miles per hour, the difference between driving into the wind and driving with the wind can be over 6 mpg. Once I got 41 mpg going one direction and only 35 mpg going the opposite direction, with a wind of less than 10 mph.  

4.     We would not be surprised that it takes more fuel to drive uphill than downhill. In Iowa, very little of the land is as flat as a billiard table. Mostly, we have gentle, rolling hills. You spend about a mile gaining 25 feet of altitude, and then spend the next mile losing it.   The fuel efficiency on this kind of terrain is no different than that of driving on a perfectly flat area, because what you lose going up these gentle hills, you gain going down.   But what if two cities are at different elevations?   What if two cities, which are twenty miles apart, have a difference in elevation of 100 ft.  (That would be an average grade of only 5 ft per mile.)  Would the effect of such a trivial grade really be measurable? Indeed it is.  Iowa City is 75 miles from where I live, and it is 160 ft lower in elevation. That’s slightly more than 2 ft/mile drop, and yet, even with no wind, I get at least 2mpg better mileage going down to Iowa City than coming back.  Why?  Try to visualize a huge hoist capable of lifting your car 160 ft in the air.  How much energy would it take to operate such a thing?

5.     But now, the real killer of fuel efficiency: the air conditioner!  You can have a drop of 5 or 6  mpg by just turning on the air conditioning.

            In the old days, we never noticed much difference in mileage no matter how we drove our cars.   No matter what we did, the mileage was uniformly horrible.  The reason was that the main waste of energy was the friction loss required to spin those monstrous engines. Compared to that, all other energy loss was trivial.   But the more efficiently we build our engines, the more noticeable the smaller losses become. And this is a high class of problems.