Doctor Diesel

Cruising and choosing

I may be able to make a small offering towards the interesting subject of the most economical cruising speed, in so much as my niece is married to a drivetrain design engineer for Bentley.

His area of expertise starts with the engine and ends with the road wheels. He was most definite when I asked almost the same question (what engine speed is the most economical to run at?)

His reply was “the speed of maximum torque”. I see you quite rightly state an engine speed range, due to the torque characteristics for a diesel engine being fairly flat. Due to the drag effect of a car shape depending upon the velocity squared of the vehicle, whether it be the most slippery shape or like a house brick, comparing 60mph with 70mph we are looking at a ratio of 36 to 49.

So to increase the speed from 60mph to 70mph, the car is using 36 per cent more fuel. In the Matthew Birkinshaw reply, your 1,800 rpm as per your example will be the most economical road speed on the flat in top gear, or even better with a tail wind.

In passing, some years back there was a picture of, I think, a Vauxhall Carlton on the front cover of DC, and it stated 17.8mpg at 120mph. Wow!

R Jewiss

Thanks for your further input Robin. There was little doubt in my mind at the time of writing that the maximum torque speed was the most economical engine speed. But there are other factors, often demanding a choice between fifth and sixth gear, as I mentioned.

If one is manually overriding an automatic gearbox with lots of ratios, such as the recently introduced eight-speed ones, the choice is even more complex. With the Jaguar XF we are told that the automatic transmission is economy optimised and the transmission will, left to its own devices, supposedly choose the most economical gear, whatever the situation. So you may as well leave manual override alone, in theory.

A similar situation applies, one might think, to gear change prompts displayed on the instruments, although again I do have some doubts on this. A few years back, somebody tested these on a then current Volkswagen Golf and found that over the EC Test Cycle, changing up earlier gave better fuel consumption than following the instrument prompt.

Whether that proves anything, in view of the nonsense of the existing EC Test Cycle, I know not, but it makes me cautious. Then there’s another complication: all the rules and conclusions generally relate to maximum economy being at the maximum torque engine speed. But that figure and engine speed is obtained at full throttle!

Do the same rules apply on a light throttle? There are complex fuel consumption maps for engines that plot out the fuel consumption at different engine speeds, and different power outputs, and only they really present the full picture.

At a given throttle setting, the best torque output can well be significantly different from that at full throttle. Final thoughts on this though. Is a Volkswagen Golf 2.0 TDI 150, with maximum torque quoted as 236lb ft at 1,750 to 3,000rpm, most economical at 1,750rpm in fourth, fifth, or sixth gear, at speeds of either 43mph, 55mph, or 65mph?

I know that we all accept that highest gears are more economical, at any given speed, but with a choice of cruising speed, and no more economical, at any given speed, but with a choice of cruising speed, and no money to fill up the tanks, 55mph might be the best buy to avoid walking home.

Quite enough of this for now, but this is evidently a rather complex subject.


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