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[colalowe]

Does anyone know what the most economical cruising speed is for a 2.5 v6?

comming back from Blackpool i noticed that 80 was showing more economical than 70?? :-?

[Bo Bo]

I've had that too, guess it depends on road conditions.....

[Kevin Wood]

It's difficult to get a good idea from the instantaneous figure. I find mine generally gets more thirsty the faster I go. Give it a good run at a constant speed after resetting the average and repeat at a few speeds.

Mine does about 33 MPG at 70, down to about 30MPG at 80, 26 MPG at 90, 22 MPG at 100 . All verified during my recent holiday, officer!

Did a run back from Bristol on the M4 the other day, in no hurry, and at 60-65 it averaged 40 MPG over nearly 100 miles.

All figures from the MID, which seems to be optimistic perhaps to the tune of 1MPG so not far from the truth.

So, slower is generally better although an auto box will suddenly get very uneconomical when you can't maintain sufficient speed to keep the torque converter locked up in 4th. About 50 MPH in my case.

Kevin

[The Cambelt Kid]

I seem to be better off at 80ish (on a runway).  Isn't it because the torque on the v6 is higher? hence the 56.5mph in a standard 4pot theory doesn't apply!

[Markie]

yep, ive always thought that breaking the speed limit in the omega is economical....

[Kevin Wood]

Aerodynamic drag increases with speed squared so, once that becomes the dominant factor in the equation, your fuel consumption increases with the square of the speed in order to overcome the drag.

However, at low speeds other factors take over. Rolling resistance and efficiency of engine and drivetrain start to become more important. Clearly the "slower is better" rule falls over when you have to change down a gear because the engine is now spinning faster with respect to your forward speed, and is therefore less efficient.

The 56 MPH speed for best consumption was always a myth, but a slightly self-fulfilling one in that 56 MPH was the speed at which the old style government figures for cruising MPG were measured, so, as a manufacturer, if you were going to tune your car for fuel consumption, you made sure it was good at 56.

Personally I think you'll get the best MPG at the lowest speed at which you can comfortably cruise in top gear, with the torque converter locked if an auto, without labouring the engine. Say about 2000 RPM in a V6 and maybe 2500 in a 4 pot. Pretty close to the mythical 56 MPH figure, then.

Kevin

[theolodian]

Aerodynamic drag increases with speed squared so, once that becomes the dominant factor in the equation, your fuel consumption increases with the square of the speed in order to overcome the drag.

However, at low speeds other factors take over. Rolling resistance and efficiency of engine and drivetrain start to become more important. Clearly the "slower is better" rule falls over when you have to change down a gear because the engine is now spinning faster with respect to your forward speed, and is therefore less efficient.

The 56 MPH speed for best consumption was always a myth, but a slightly self-fulfilling one in that 56 MPH was the speed at which the old style government figures for cruising MPG were measured, so, as a manufacturer, if you were going to tune your car for fuel consumption, you made sure it was good at 56.

Personally I think you'll get the best MPG at the lowest speed at which you can comfortably cruise in top gear, with the torque converter locked if an auto, without labouring the engine. Say about 2000 RPM in a V6 and maybe 2500 in a 4 pot. Pretty close to the mythical 56 MPH figure, then.

Kevin

Yeah, mine gets over 40 at 65 (instantaneous over smooth flat ground).  On the Motorway, over 75 does cause a drop.  Depends though, and accelerating too slowly is actually worse on MPG.

[cem_devecioglu]

I measured

7 liter/100 km at 100 km/h average speed

9 liter/100 km at 140 km/h average speed

ps: resistance of fluids (also air) increases squared in proportion to velocity in the fluid

 :-/

oof bad english

[Ken T]

I used to do the morning run to Oldham, along the M62, at about 70ish, and got about 30mpg (lots of stop/start). I started taking it easy, sticking to 56-60, not hammering up hills, and get an extra 12% MPG. On a £65 fill up, that equates to an 8 pack of Export free, just for driving smoothly!