Hypothetical mpg question fo a 90 mile journey.

anotherbaldrick

Put it this way. At 90 mph my 3 litre TDI in 6th would be turning over at 3600 rpm, it is cranking 2800 rpm at 70 mph. It red lines at 5000rpm at 125 mph.

Norman_Castle

reeac wrote: »

Not true, the increased drag at the higher speed will require a bigger throttle opening [or diesel equivalent] and hence use more fuel.

Ignoring the variables of friction, load, wind resistance etc which will have a limited effect, an engine running at 2000rpm will use the same amount of fuel at any speed. The distance and engine speed are fixed. The only variable is the time taken due to the gear used.

Retrogamer

anotherbaldrick wrote: »

In 4th gear at 2000 rpm you would not be doing 30 MPH

You would be in my old car.
In my new one you'd be doing 2000rpm in 3rd gear at 30mph IIRC

anotherbaldrick wrote: »

Host economical would be highest gear at 1200 rpm

Unless that due to the lack fo power / torque at such a low RPM you are "labouring" the engine and use more fuel than a lower gear, at a higher RPM when the engine isn't under as much load.

Ultrasonic

Norman_Castle wrote: »

Ignoring the variables of friction, load, wind resistance etc which will have a limited effect, an engine running at 2000rpm will use the same amount of fuel at any speed. The distance and engine speed are fixed. The only variable is the time taken due to the gear used.

What you're ignoring is VERY significant, not trivial. To suggest that a vehicle will use even approximately the same fuel at a particular rpm no matter what else is happening is utter nonsense. By that 'logic' it would take as much fuel to drive downhill as uphill, which it certainly doesn't. Even on the flat it is just plain wrong to suggest this.

The answer to the original question is pretty much bound to be doing the trip at 30 mph in 4th gear due to aerodynamic drag.

reeac

Norman_Castle wrote: »

Ignoring the variables of friction, load, wind resistance etc which will have a limited effect, an engine running at 2000rpm will use the same amount of fuel at any speed. The distance and engine speed are fixed. The only variable is the time taken due to the gear used.

How can you ignore those factors? When at constant speed those are the only things requiring fuel useage [OK maybe the heater and A/C and wipers if it's raining.].

Norman_Castle

I'm ignoring the variables because they are not given and are potentially endless. Is this a Luton van with a low torque engine on a windy day which only maintains 90mph with the throttle on the floor or an aerodynamic car on low resistance tyres fitted with an engine producing maximum torque at 2000 rpm and ideally suited to 90 mph cruising ?

londonTiger

60-65mph gets you the best mpg figures. This is partly why lorries are limited to that speed.

Any faster and the drag pushes back harder and causes the car to use more fuel per distance covered.

paddedjohn

Interesting answers, the actual figures for my van are more like 80mph in 6th gear @2000rpm and 35mph in 4th gear @2000rpm but I chose the earlier figures just to simplify the calculations.

spacey2012

The fastest way to get a ticket is to try and drive in a 30 limit in 4th gear.
Thats why the car has a gearbox, it is easy in third, near damned impossible in 4th

Ultrasonic

Norman_Castle wrote: »

I'm ignoring the variables because they are not given and are potentially endless. Is this a Luton van with a low torque engine on a windy day which only maintains 90mph with the throttle on the floor or an aerodynamic car on low resistance tyres fitted with an engine producing maximum torque at 2000 rpm and ideally suited to 90 mph cruising ?

If you were a newbie I'd assume you were trolling.

The difference in air resistance between 90 mph and 30 mph is a HUGE factor, you simply cannot ignore it.

(You could start arguing about the likely higher engine efficiency at the higher load in 6th at 90 mph vs 4th at 30 mph, but it would have to be one hell of a weird (frankly badly designed) engine for this to get anywhere close to offsetting the aerodynamic drag effect.)