Average speed

[Segilla]

I drive moderately so could eaily get up to 62 mpg** in my former 1.9 diesel Skoda.
But the engine may consume more fuel on lengthier journeys compared with identical faster ones.
I doubt there is any formula which can help to work out which is more economical, but maybe someone has a comment about this.

** including habitual coasting; My 1.6 petrol NOTE achieves a disappointing 51 mpg.

[jake-the-peg]

Fuel efficiency drops off very rapidly with speed - As I recall the wind resistance is proportional to the cube of the speed so you get a curve like this:

http://www.drive55.org/downloads/tdiclubchart. gif

you also should check out point 102 here
http://www.highwaycode.gov.uk/09.htm

[Teddio]

Your Skoda probably has the 1.9TDI diesel engine. Maximum engine torque is developed between 1800 and 2400RPM. In top gear at 60mph the engine speed is 2000RPM which is near its maximum efficiency.
Most of the counter force on a car is air resistance and most of the work done by the engine simply pushes aside the air. If you double the speed of your car you quadruple the air resistance. The most efficient speed for your Skoda diesel is probably around 60mph.
Diesels are inherently more efficient than petrol cars on account of their greater compression ratio and can convert approximately 40% of the chemical energy in the fuel to useful work (nearer 20% for petrol engines)