Q: When running very low on petrol while driving on a motorway, what speed and gear would generally give the best mileage per gallon in order for you to reach the nearest service station?
A: On a motorway, speed affects fuel economy in two ways: through energy loss due to friction of components and through energy loss due to wind resistance. Generally, as a proportion of total energy loss, losses due to component friction are inversely proportional to vehicle speed, whereas losses due to wind resistance are directly proportional to speed. Although other factors such as the slope of the road come into play, achieving maximum fuel economy is dependent upon finding a compromise speed at which these losses are reduced to a minimum.
A: The best figures will be achieved by engaging the highest gear ratio and driving at the lowest possible speed, in that gear, without labouring the engine. Between 65 and 110 kilometres per hour there is an approximately linear decrease in kilometres per litre as speed is increased. Hence, the slower you drive the less fuel is consumed per kilometre. The highest gear ratio will give the greatest number of road wheel revolutions for the minimum number of engine revolutions.
Advertisement
A: Competitors on fuel economy drives use the highest possible gear and the lowest allowable speed. This minimises drag forces and avoids wasting power that is used to simply turn over the engine and transmission. However, the rules of such competitions preclude coasting and there is a minimum average speed.
Without these restrictions, the most economical method is to pick a minimum and a maximum speed (say 65 and 95 km/h) and accelerate at full throttle from the lower to the higher, then allow the car to coast until it has slowed to the lower speed, then accelerate again, repeating this process for the whole journey. In this way, the engine always operates at maximum efficiency.
The two speeds chosen depend to some extent on the drag coefficient (which can be substantially lowered by overinflating the tyres so they are very hard) and the range over which the engine develops its maximum torque. Always depress the throttle pedal gently, as a sudden flooring pumps extra fuel into the system, enriching the mixture unnecessarily. I have tried this method (on long night journeys when the roads were empty), and it can save up to 40 per cent on fuel consumption.
A: How far you will get in a car running out of fuel depends on the engine size, the speed and the fuel. Fuel consumption (litres per kilometre) is optimal for the following combinations: 1600 cc engine running on diesel at 60 km/h; 1600 cc on petrol at 70 km/h; 1300 cc on petrol at 80 km/h. Top gear is the best for all these combinations and the speeds are for engines in perfect condition. Extra wind resistance from items such as suitcases and bicycles on the roof will tend to reduce the optimal speed (see graphic).
A: When driving at a constant speed, the lowest consumption is obtained at minimum speeds. The system used by drivers in mileage marathon competitions is called 鈥渋mpulse-acceleration-impulse鈥�. Accelerate up to 30 km/h in fourth gear, then cut the ignition and change the gear to neutral. Let the speed go down to between 8 and 12 km/h and start the engine again, repeating the operation. This method should not be used if your steering wheel is locked when you cut the ignition.
A: Careful reading of the road ahead, coupled with measures such as accelerating briskly down hills and throttling back to travel up them can increase your range by as much as 10 per cent. As a further measure, and if you have the requisite amount of bravery/stupidity, you could sit a few feet behind a large vehicle to minimise air resistance. Just make sure it isn鈥檛 mine.
Don鈥檛 try the techniques described in these last three letters on public roads 鈥� Ed