Making the fuel go further

 

A couple of weeks ago The Guardian published some tips on coping with the soaring petrol prices. One suggestion was to stop driving at 80mph on the motorway but drive at 70mph instead, making a 20% saving. That's no use to me: it's illegal to drive at 80mph so I don't. But I was interested in where the statistic came from. I was going to do a full calculation based on the drag equation but then it occurred to me that when comparing between two speeds, the other terms should be the same and I just need to consider speed. I am, of course, assuming that all fuel consumption is proportional to drag. Drag is proportional to speed-squared. So I did 80^2/70^2 and got 1.31 so the fuel use at 80mph would be 1.31x higher than that at 70mph. So isn't it a 30% reduction? I normally do percentage difference as difference/true answer. In this case it might be (80^2-70^2)/80^2 = 23%, being 23% of what it WAS at 80mph. That's probably where the figure came from. Now trying a reduction from 70mph to 60mph, I get a 27% reduction! But I remember from the oil shock of the 1970s that 55mph is said to be the most fuel efficient speed. This site gives some figures to consider.