Driving an hour to and an hour from work every day has caused me to become borderline obsessed with the fuel efficiency of my car. A few weeks ago I flipped through the manual, found the capacity of my gas tank, drove to and from work until it was nearly out of gas, filled it up again, and compared how much gas was left to how many miles I had driven (for the more science-y guys here, is that rigorous enough? I wasn't sure if I was leaving something out). Through all that I got a rough picture of the miles-per-gallon on my car: about 30 on a good day, not amazing considering it's a Honda Civic driving mostly on the highway; not horrible when you weigh in on the car's age ('93, with more problems than I have money to fix). It may be an unfair comparison, but just for fun the 2010 Prius or the Civic Hybrid got about 40-50 just on city driving. Sounds pretty nice right now...
However, through all that I was still worried that I wasn't being thorough enough. Maybe the gas station pump shuts the gas off before the tank is full, maybe there's more gas left than I think when the needle reaches E (or less than I think when it says full), maybe the air pressure in the tires isn't exactly where it needs to be, etc. Well, seems like a few of my concerns were justified when I stumbled on to this today. Even though everything else on our cars has been computerized and improved over the years, it seems that people like the illusion and comfort of knowing that our tanks have a little extra fuel in them than what the gauge tells us despite the fact that it messes with just how fuel efficient your car really is (kind of like the illusion of more safety on airplanes by adding more and more inefficient security). Things apparently get even more confusing if you top off at the gas station, something I don't do but according to the article can add almost another full gallon of gasoline. All-in-all it's not that big of a deal, but I always wondered why they didn't just put a digital number there that tells you how much gas is left. I guess nobody likes to hear the truth.
I don't know if this actually ends up improving my car's fuel efficiency or not. I just don't know what to think knowing that my fuel gauge is lying to me. I thought we were cool, gas tank...
Whenever I calculate fuel efficiency I ignore the gauge altogether. I first fill the tank until it shuts off (i.e. I don't "top off"); I then reset the trip mile counter. The next step is easy, too — drive! The more gas you burn through, and the more miles you drive, the more accurate this will be. Generally more than 3/4 of the tank is good, more than half is fine. Less than half and you probably have driven considerably more city than freeway. or vice versa, thus yielding biased results. Okay, so I don't completely ignore the gauge. Then after all that driving and gas burning, it's time to refill. Again fill to the first shut off point. Now take the miles driven, 246.7 mi for example, and divide by the gallon count on the gasoline pump, 8.973 gal. This gives 27.49359 etc. etc. etc. so 27.5 mpg.
ReplyDeleteOn a related note to tank gauges, since the E orange line signals 1.0 gal or there abouts, or even better if the car has a low fuel light, which also signals about a gallon of gas, knowing the avg fuel economy, 27.5 mpg, and rounding down, 25 mpg, you now can estimate how many miles you have to find the next gas station. (25 miles hopefully)
That's how I roll.