I've read your pages in the Telegraph for years but have never written before so I will take the opportunity to say I think you do a great job. Regarding the calculation of miles per gallon, the most accurate way of calculating the average real miles per gallon achieved is to keep a note of the cumulative miles driven (from the car mileage clock) and the cumulative number of gallons brought (by multiplying litres by 0.22). When five or more data points are available, these are plotted on a graph and the miles per gallon is the gradient of the graph (a spreadsheet is the best tool for this). (Yes, I am a professional engineer.) The key advantage of this method is that "tank fraction filled" variations are averaged away over several tank refills.

I have used this method for years and have found it precise enough to see reductions in fuel economy due to the need for car maintenance and (I think) I can see that fuel economy is better in summer than in winter (due to the engine needing less time to reach its correct operating temperature). Over 30,000 miles, my Audi A3 Sportback (1.6 petrol) has achieved 42.2mpg. This has been achieved by the diligent application of all the tips you promote (regular maintenance, no short journeys, minimum weight, correct tyre pressures, reading the road, etc.) but also includes travelling at up to 70mph whenever safe and legal to do so.