Hypothetical mpg question fo a 90 mile journey.

paddedjohn

90 mph for 1 hour in 6th gear @ 2000rpm.
or


30 mph for 3 hours in 4th gear @ 2000rpm.


would there be a big difference in fuel consumption?


Only wondering because the tv is crap tonight.

DUTR

Well the 1st journey would possibly gain you a ticket in the UK.
At the higher speeds the drag would increase, so technically the 30mph for 3hrs, with gentle acceleration and easy braking.

Norman_Castle

2000rpm for three hours would use three times as much fuel as 2000 rpm for one hour, with adjustment for wind resistance. It would also be very boring.

reeac

Norman_Castle wrote: »

2000rpm for three hours would use three times as much fuel as 2000 rpm for one hour, with adjustment for wind resistance. It would also be very boring.

REmember that it's a 90 mile journey and we're talking gallons/mile not gallons/hour. Since power required increases as the cube of speed we're talking about an increase in power required of X27. In practice it wouldn't be such a big effect as the power train would be less efficient in 4th. gear than in 6th. Overall, probably not too much difference in fuel consumed.

anotherbaldrick

In 6th gear at 2000 rpm you would not be doing 90 MPH.
In 4th gear at 2000 rpm you would not be doing 30 MPH


Host economical would be highest gear at 1200 rpm


I agree TV is crap tonight Just put something in the microwave and watch it go round .

WTFH

anotherbaldrick wrote: »

In 6th gear at 2000 rpm you would not be doing 90 MPH.
In 4th gear at 2000 rpm you would not be doing 30 MPH


Host economical would be highest gear at 1200 rpm

How can you make those three blanket statements? You don't know the engine, the fuel, the power output or the gearbox.
I agree that it is highly unlikely that an engine which is 45 mph/1000rpm in 6th would be 15mph/1000rpm in 4th, more likely to be 3rd or maybe 2nd.
1200rpm might not be the most economical. Perhaps it is in some cars, but not in all.

anotherbaldrick

WTFH wrote: »

How can you make those three blanket statements? You don't know the engine, the fuel, the power output or the gearbox.
I agree that it is highly unlikely that an engine which is 45 mph/1000rpm in 6th would be 15mph/1000rpm in 4th, more likely to be 3rd or maybe 2nd.
1200rpm might not be the most economical. Perhaps it is in some cars, but not in all.

Can you be positive to the alternative, you have already expressed severe doubt

Norman_Castle

reeac wrote: »

REmember that it's a 90 mile journey and we're talking gallons/mile not gallons/hour. Since power required increases as the cube of speed we're talking about an increase in power required of X27. In practice it wouldn't be such a big effect as the power train would be less efficient in 4th. gear than in 6th. Overall, probably not too much difference in fuel consumed.

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.

Norman_Castle

anotherbaldrick wrote: »

In 6th gear at 2000 rpm you would not be doing 90 MPH.
In 4th gear at 2000 rpm you would not be doing 30 MPH

Its a hypothetical gearbox.

[Deleted User]

My TV is never on at the weekend.

reeac

Norman_Castle wrote: »

an engine running at 2000rpm will use the same amount of fuel at any speed. The distance and engine speed are fixed.

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