Is 56 MPH the most economical speed?

[screem]

Or does it depend on other factors such as the size of engine etc?

[balls-out]

It will depend on.
engine size
engine tuning
gearing
cd of the car
frontal area of the car

[hairykrishna]

Depends on engine, gearing and aero. I think it's normally around that speed though.

[Paul_M3]

56 mph is 90 kph, which is the figure that is often quoted for ecomony.

People see 56mph and think it's a special number, when in fact it's just a conversion of a sensible round number in kilometers which is used in Europe.

Edited by Paul_M3 on Monday 29th March 18:30

[Glosphil]

Uphill? Down hill? Into a strong wind? Strong tail wind? Too many varying factors, including those given above, to generalise.

[kambites]

Every car I've ever owned has had an optimal fuel economy speed of much lower than that. It's hard to be certain without a trip computer, but the Elise seems to be most economical at around 25mph in third or 35mph in 4th. The Punto reads highest on the trip computer at around 20mph in third and I have no reason to disbelieve it.

That's all on the flat with no wind.

Down hill the answer is obviously "as fast is it rolls with the engine switched off".

Edited by kambites on Monday 29th March 18:35

[screem]

Glosphil said:

Uphill? Down hill? Into a strong wind? Strong tail wind? Too many varying factors, including those given above, to generalise.

Assuming a flat road, no wind and im driving a 4.2 supercharged Range Rover sport?

[kambites]

screem said:

Glosphil said:

Uphill? Down hill? Into a strong wind? Strong tail wind? Too many varying factors, including those given above, to generalise.

Assuming a flat road, no wind and im driving a 4.2 supercharged Range Rover sport?

Ask the trip computer.

I'd guess somewhere in the 20-30mph range though.

[screem]

balls-out said:

It will depend on.
engine size
engine tuning
gearing
cd of the car
frontal area of the car

Engine size: 4.2 Petrol
Tuning: Standard
Gearing: standard
Cd of the car: Normally the Stone roses and occasionally Queen Greatest Hits (only joking) I have no idea.
Frontal Area: No Idea

[screem]

kambites said:

screem said:

Glosphil said:

Uphill? Down hill? Into a strong wind? Strong tail wind? Too many varying factors, including those given above, to generalise.

Assuming a flat road, no wind and im driving a 4.2 supercharged Range Rover sport?

Ask the trip computer.

I'd guess somewhere in the 20-30mph range though.

The trip just says 14.5 Miles per Gallon Average in the last 3 months and an average speed of 21.6 mph ?

I do a 50/50 split of m/way and City.

[kambites]

If you're averaging 21mph, I'm guessing that you're spending quite a lot of time stationary or moving very slowly.

[ShadownINja]

Always wondered about this. 56mph was the quoted speed back in the '80s when technology was a bit black and white. Is it really still the same?

[SClarke]

I would imagine for most normal cars top gear and around 1000rpm would prove the most efficient.

[kambites]

SClarke said:

I would imagine for most normal cars top gear and around 1000rpm would prove the most efficient.

Why top gear? Modern engines don't have very much internal friction so the efficiency advantage from higher gears isn't huge. Don't forget that aerodynamic drag increases much faster than linearly.

Also, in a petrol at least, 1000rpm is typically too low. Both of my cars seem to over-fuel if you let them drop below about 1700rpm.

Edited by kambites on Monday 29th March 19:26

[Kevin VRs]

I believe the 90kmh/56mph figure is quoted because it is the rural speed limit on single carriageway roads in most of Europe.

[The Lukas]

I always found a constant speed of 40mph (where appropriate) was very economical. In top gear of course. You must balance efficiently with effectiveness however.

[300bhp/ton]

screem said:

Glosphil said:

Uphill? Down hill? Into a strong wind? Strong tail wind? Too many varying factors, including those given above, to generalise.

Assuming a flat road, no wind and im driving a 4.2 supercharged Range Rover sport?

If you're worries about fuel economy and you are driving that, then seriously get a different vehicle!!!

I see no point in people buying big engined cars and then driving them like a nampy pampy baby. Swap it for a diesel, drive it hard, it'll be faster more fun and still see better mpg than the 4.2 ever will.

Either that or make the decision that you don't give a damn and just enjoy it and not worry about the fuel.

[Marquis_Rex]

screem said:

balls-out said:

It will depend on.
engine size
engine tuning
gearing
cd of the car
frontal area of the car

Engine size: 4.2 Petrol
Tuning: Standard
Gearing: standard
Cd of the car: Normally the Stone roses and occasionally Queen Greatest Hits (only joking) I have no idea.
Frontal Area: No Idea

That's the AJ33 supercharged Jag engine- all in all- with very good efficiency numbers at part load- shame it's in a heavy unaerodynamic brick

[otolith]

Paul_M3 said:

56 mph is 90 kph, which is the figure that is often quoted for ecomony.

People see 56mph and think it's a special number, when in fact it's just a conversion of a sensible round number in kilometers which is used in Europe.

This is the answer. There's nothing at all special about 56mph, it's just that when the numbers quoted were for "urban", 56mph and 75mph, 56mph was always the best of the three.

[eldar]

screem said:

Or does it depend on other factors such as the size of engine etc?

Turning the engine off and doing 0mph is fairly economical