What is the most economical speed to drive at?

[T & P]

Not that I'm itching to do it but if I wanted to go from A to B for the least fuel consumption what would be the best speed at which to do so? Assuming I had a choice of motorway, A and B roads?

Before they introduced standard consumption figures throughout the EU, car makers in Spain used to give their best fuel consumption figures at 90 kph (56 mph). Is that the best speed then? I would have thought with modern cars it'd be around 60 mph, specially if it has 6 gears.

 

[chymaera]

Not that I'm itching to do it but if I wanted to go from A to B for the least fuel consumption what would be the best speed at which to do so? Assuming I had a choice of motorway, A and B roads?

Before they introduced standard consumption figures throughout the EU, car makers in Spain used to give their best fuel consumption figures at 90 kph (56 mph). Is that the best speed then? I would have thought with modern cars it'd be around 60 mph, specially if it has 6 gears.

It depends entirely on the vehicle and if the engine is petrol or diesel.

 

[T & P]

Sure, but let's say it's petrol and in the 1.4 to 2.0L range. I don't want a close answer, more like a ballpark figure (for instance 40mph in 4th gear vs. 60mph in 5th or 6th gear if present).

 

[Velouria]

Sure, but let's say it's petrol and in the 1.4 to 2.0L range. I don't want a close answer, more like a ballpark figure (for instance 40mph in 4th gear vs. 60mph in 5th or 6th gear if present).

The question is still too vague to be answered with any degree of precision

There are a huge number of factors ... the 'tune' of the engine (high torque or high rpm?), the aerodynamics of the car, the terrain of the journey, the gearing of the car, the size of the tyres, the drag of the brakes, the age of the engine, the mass of the car and load, the traffic flow ... just to mention the major factors ...

Even the weather and altitude can affect your performance! (density of air and petrol changes...)

 

[chymaera]

The question is still too vague to be answered with any degree of precision

Precisely . For instance some diesel engines are designed to run flat out to get the best fuel economy.
All the "destructions" on out Renault Clio petrol engine, automatic gearbox states is the best economica speed for the engine is 3000rpm. (As it is an Automatic gearbox 3000rpm is quite a variable speed range.)

 

[muckypup]

at walking speed, with you behind the car pushing it. also lends itself towards environmental sustainability.

 

[two sheds]

at walking speed, with you behind the car pushing it. also lends itself towards environmental sustainability.

Yep:

1) switch engine off
2) that's the most economical speed

 

[zoltan]

people assume that 56mph is the most efficient speed - it not - rather 56mph = 90 kmh - its just an nominal A road cruising speed used to compare car to car in testing & quoted in figures

best fuel economy varies from car to car according to every possible variable you can think of

 

[ICB]

Usually the lowest RPM in top gear at which you have a sensible amount of torque. What speed that is depends on the engines torque curve, 'box ratios and final drive ratio. Coefficient of drag is a big issue as well.

Maintaining a constant throttle position is more economical than maintaining a constant speed (i.e. giving it more for climbs and backing off on descents)

 

[Chz]

I'd heard it said in the past that it was the lowest RPM in top gear where you'd get ~80% of your torque. From a purely mechanical standpoint, it would be the torque peak but aerodynamics and other things come into play.

In *most* cars, it's in the 55-70 range. But it can obviously vary wildly depending on the engine and the gearing. If you've got a big engine or a diesel with lots of torque, it's the lowest speed you can manage in top gear without lugging the engine. My old 635 was most efficient at 55 or so, but fuel use changed very little up to about 75. (where areodynamics really murdered it)