do you all actually work out your MPG or just estimate it by how many mile you get to say £10?
Good question lee. The only way you can get an idea is from fillup to fillup. The electronic gauge on the dash usually over reads and so does the odometer.
My trusty 1988 2.0l Ford Sierra (petrol) GLE in South Africa was an eye opener. Not only did the speedometer over read by 10% but so did the odometer on a measured 1 km. (there was a section of road in Johannesburg not far from us where this was measured and marked.)
This meant that not only was our fuel consumption 10% better than it actually was, but also meant that when the car had done 100k it actually had only done 90k. It also meant that Ford (if main dealer was the chosen service agent) got another service out of it - like the difference between monthly and 4 weekly cleans.
But it gives an idea of whats happening. Both our Hdi vans use more diesel on cheap supermarket fuel when compared to Esso and Shell. I can also tell exactly what fuel is in the tank when I come up Saltburn bank with a fullish tank of water.
Tyres are also another factor. HGV drivers get their truck's speed limiter recalibrated with 'bald' tyres, so when they fit new ones they can go fractionally faster.