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Hi there JordyLad, and welcome to the forums.
Good question - the classic 56mph figure was just a benchmark used in the government road testing, and for comparative purposes only.
The most 'efficient' speed will vary from car to car, and even for engine/transmission type, but a general rule of thumb method is;
- look up your owner's handbook and locate the power characteristics for your specific car.
- find the maximum torque figure, and what rpm it is developed.
- if your engine is at that rpm 'sweet spot' in any gear - you are at the most fuel efficient mode that the car has.
The reasoning is simple, use too many revs - then the fuel is being burnt more rapidly than is necessary, change up too early, and you will be forever changing down to maintain progress (remember, every time you change gear, the engine is still using fuel). At the max torque engine speed, you shouldn't have to change down (unless you encounter an obstruction).
Hope that helps, try it out and see how you fare.
This job should never be complete - the day I think I've done it all, is the day I resign.
Currently in VelSatisfied's garage:
2005 FIAT Ducato 2.8JTD LWB Gran Volume, 2000 Kawasaki ZZR 1100 D7 'fullpower' in black + full GIVI & KAPPA luggage, 2007 MB R-Class 320 CDi Sport LWB uprated by Brabus to 300Bhp don't know if there's a 155mph limiter - will be fun finding out! 2004 Mercedes Vaneo 1.7 CDi Ambiente 7-seater