As you know I like to keep track of my gas milage. The past few weekends I have been traveling and therefore had the chance to get some highway driving instead of the usual commuting. I’ve left the average consumption up on my OBC to monitor my better-than-usual milage and also remind me not to put the pedal to the floor as much as usual, in the hopes of breaking my seemingly unobtainable record of 28mpg. After two fill ups, all the OBC really gives me is false hope. My last fill up, the OBC reported 28.6mpg when calculated miles/gallons was only 24.6. I thought I would give it one more chance in case of extra gas being pumped into the tank so I waited till today’s fill up: OBC reported 29.4 and real milage was 24.5. That’s nearly 5mpg overly optimistic!
Now it is understandable, the OBC’s data most likely comes from the sum of pulse widths of fuel injectors, and fuel measured of a few microliters per pulse, then figure at up to 6500 rpm that is up to 13 thousand injections per minute…so as you can imagine there can easily be a large margin of error! And although I have not watched it as closely, I have always noticed the OBC’s readings being higher than my actual milage, but usually by more like 2 or 3 mpg (and perhaps the upgraded JCW injectors add more to its confusion).
The point is, don’t go assuming you are getting the milage that your OBC is reporting. Not to mention the fact that most of our speedometers read higher speeds then actual; compared with more accurate GPS-determined speeds, mine will read 68 when actual speed is 65. So that means the odometer is reading further than you’ve actually travelled, so actual performance is even worse!
Needless to say I do not advertise my car as a gas-saver. Squeezing 210hp out of a 1.6 liter engine, combined with the high average speeds of Mass’s highways and my hilly stop-and-go commute, my usual milage is probably one of the lowest for MINIs. But it is fun to keep track of and think about gas economy once in a while, especially when prices are this high.