When should I change to fifth?

QThere’s a long, fast road near my house that seems to involve constantly changing between fourth and fifth gear in my Astra. A friend suggested that it’d be more economical to just use fourth. But surely using fifth as much as possible is a better idea? What do you think?

Luke Buchanan, Hastings

AWe conducted some trials using a device that directly measured the fuelflow into the carburettor. If the car was doing a speed where it could operate in either of the two gears without strain, the fuel flow was more-or-less the same in either. Choosing the higher gear would make the most of the small difference in consumption on a long trip.

However, we noted there was a significant fuel penalty for changing gear, caused a small loss of momentum and the need to step on the throttle immediately afterwards to maintain a constant cruising speed.

So – if the car can be left in fifth for many miles, by all means use it. If it can barely cope and has to be dropped back to fourth, don’t bother. Labouring the engine at low rpm causes far more longterm damage to it and to the gearbox than using a lower gear at higher rpm.