Trip Computers

One automotive gadget which I really find to be of use is the trip computer. I’ve had this on a few cars now and have found the average fuel consumption feature to have a big impact on driving style. Essentially, if I can see the mpg figure going down, I know it’s time to watch my speed and keep the occasional boy racer tendencies in check. Whether this makes me a safer driver or not is debatable but it certainly makes me a more efficient one which is just as well considering the cost of fuel these days.

I have always found the fuel consumption figure on my car to be pretty accurate. This doesn’t always seem to be the case and I know that my previous car, a Honda Jazz, would regularly over estimate how many miles I was getting out of a gallon of fuel. Having had a straw poll around the office it seems to be a common occurrence: “it says 45 mpg but it never gets more than 40” is quite a typical response. There is no legislation on the accuracy of trip computers (unlike speedometers which must be within a specific tolerance and never under-reading) but I would expect manufacturers to be able to come up with something fairly close to the truth. Being somewhat of a geek I record both my actual and indicated mpg via the Fuelly website. I’ve plotted both the actual (measured at the pump and on my odometer in blue) and trip computer (in red) over the period of a year:

It’s actually remarkably accurate. In fact the average mpg measured at the pump is 52.4 for that period compared to 51.9 according to the trip computer. The trip computer is, typically, just under 1% pessimistic compared to the actual figure. I think I can live with that. In fact I noticed that one other Fuelly user, “Tezzer”, has done a similar thing but this time with the newer model Yaris:

 This is also pretty spot on but this time gives and average actual figure of 56mpg compared to an indicated 56.7. That’s 1.25% optimistic although I would say that was perfectly acceptable accuracy. Given that somewhat limited sample of two cars it would appear that Toyota are quite capable of designing accurate trip computers. So why did my sample around the office (and also on many internet motoring forums) give the impression that trip computers are wildly optimistic?

I suppose the problem is that most people do not accurately measure their fuel consumption. Aside from the two vehicles I’ve shown very few cars in real world driving conditions actually achieve what the manufacturers claim they are capable of according to the standardised emissions tests. Many people aren’t very happy about this and feel that they have been actively lied to when they found out. However, for most motorists, when asked what sort of mileage they get from their cars, will use some sort of vague “I get x miles out of a tank” or quote some unrealistic figure that they calculated on a long run whilst on holiday. The other answer, of course, is to quote the figure that the trip computer spits back at them. Given that there is no legal requirement to have an accurate mpg figure it must be quite tempting to make the gauge as optimistic as possible.