on the very edge of my memory I remember reading an article on a Honda Civic forum about how to fit a widget in the instrument wiring between the gearbox speed sensor and the odometer. Its purpose was to cancel out (say) one in 10 impulses and provide a lower speed reading. It was adjustable.

I think most car odometers function on the same principle (i.e. counting impulses from a rotatonal sensor in the transmission) and in your case this data is also used by the fuel computer.

Nearly all car speedos / odometers over-read by about 10% to stay the right side of the law whilst compensating for new / worn tyres. It seemed very involved to change this as previously mentioned. I'd say either live with it and apply a correction factor in your head, or maybe consider slightly taller profile tyres which will essentially raise the gearing of the car slightly and bring actual speed closer to displayed speed. There are obviously lots of potential pitfalls to this such as performance, insurance, wheelarch clearance etc, but I've heard of it being done.

>> I wonder if the figures it shows for mileage on remaining fuel might be dubious too.

I've used brim to brim fill-ups and even when the computer showed "miles to empty" as 0 I was able to add a max of 50 litres, implying 3 litres still remaining. Having said that I've also noticed there are slight variations between one Focus TDCi and another TDCi.

Safest thing to do is a double check. Check the fuel gauge, if it shows 0 or E and the "miles to empty" is close to 0 then fill with fuel asap, ideally fill well before.