I drove up to Chicago this weekend because there was a big do at our old church with the bishop coming to visit. It was a lovely service.
I had to think a bit about going though...going on weekend road-trips has become an expensive proposition these days with gas between $4.10 and $4.25 a gallon in these parts. It is becoming painful to fill up the tank.
To save on gas, I took our 10 year old Corolla. My husband has carefully kept track of its gas mileage over the years we have had it. When we first got it in 2000, it got 32 mpg when my husband was driving on the highway (my husband drives a lot faster than I do, so I am guessing the mileage was better when I drove). Over the years it has dropped a bit to only 30 mpg. Still, that is way better than the new domestic cars I see advertised as having "Great Mileage!" when in fact they only get 26 mpg on the highway. And that is their advertised mileage...everyone knows the actual mileage will be worse.
Anyway, as I started out on this trip, I got to thinking..."just how good can I make the gas mileage this trip?". So, even though I had 3/4 a tank of gas, I stopped at a gas station very early in the trip and topped off the tank. Then I reset the trip counter on my odometer to 0. And then I began my first experience as a novice at the sport of hypermiling (ie; squeezing each gallon of gas until it screams). I don't recommend emulating a number of things described in the link...any manouever called a "death turn" designed to save a penny or two worth of gasoline defies common sense.
Away I went. I made sure that I kept careful track of my speed. As you can see in the graph at right, gas mileage is best at around 55 mph. Any faster than that, and the gas mileage dramatically worsens. Our old Corolla gets around 30 mpg when driven at highway speeds regularly reaching (or exceeding, if I know Mr.Aabsinthe) 80 mph. But the graph at right suggests that we could potentially get 25% better gas mileage if we drove at a constant speed of 55 mph. I didn't drive as slowly as 55mph for most of the trip because it is actually way too far below the speed limit of 70 mph on the freeway in these parts. I did keep it below 65 mph as much as I could though.
Also, whenever I could I drove behind semi-trucks. I didn't tailgate them, but I kept a space of about 2 seconds between us (around 200 feet, depending on the speed). According to wind-tunnel tests, drafting at a distance of 100 feet reduces drag by 40%. Getting 2 feet away reduces drag by almost 95%. The former distance is too close for my comfort, and the latter distance is suicidal. The actual drag reductions in true road-tests are not as good. I don't know what the drag reduction is at 200 feet, but I do know that when I got into that 2-second-behind-the-truck sweet spot, the amount I had to depress the gas pedal to maintain my speed appeared to noticeably decrease.
I should also note that when you are driving 60 mph in a 70 mph zone, you get way less flack from other drivers if you are behind a truck rather than toodling along by yourself. Noone tailgates you or flashes their lights if you are behind a truck.
This website calculates how much you value your life based on how close you follow trucks. My comfort distance of 180 to 200 feet or so is too large to be included in their "tailgating" calculation.
When I got back home, I topped off the tank again. My gas mileage was then easily calculated by dividing the reading on the trip odometer by the amount of gas needed to top off my tank when I got home. The result: during the trip I got 42.4 mpg!
Mr.Absinthe was favourably intrigued by this result. There is now a standing challenge with him to meet or beat that gas mileage the next time he drives to Chicago.
