[panky12345]

Originally Posted by gemithomas

Would it be possible that mileage drops if you drive sedately (around 1500 rpm) and not letting the turbo kick at around 1800? I mean is it just the opposite than petrols where you tend to get better F.E running at lower rpms

Exactly my concern.. I dont drive too fast and generally <60 at around 1600-1800 rpm.

[gpa]

Since most of my driving is within the city with a few highway trips, I generally stick to a speed of 55-65 kmph in the city and consistently get a mileage of 17.5-19 kmpl in the city with minimal use (30%) of the AC on my Alto.

Cheers,
gpa

[dakaikini]

I have been driving my Swift Dzire ZDI (Diesel) approx. 75 - 80 Km daily from the time I purchased it in June 2008. I have been getting a consistent average of 20 - 22 kmpl in Mumbai with the AC on. Of course I use the 5th gear almost always (not difficult in this car due to its high torque), switch off the engine at signals (since my path is the same I know how much waiting I can expect) and drive in slow-moving traffic by just controlling the clutch and no accelerator (again thanks to the fantastic engine torque).

A little R & D I have done shows that the best fuel efficiency in my car is obtained at 2000rpm and at a speed of 80kmph in the 5th gear.

[GTO]

Originally Posted by gemithomas

Would it be possible that mileage drops if you drive sedately (around 1500 rpm) and not letting the turbo kick at around 1800? I mean is it just the opposite than petrols where you tend to get better F.E running at lower rpms

Absolutely yes. The FE will be far superior when the turbo is on boost, rather than at an rpm where it is barely running.

[vasudeva]

Great that people are concerned about economy rather than high revving. PCRA is running ads touting 20% discount on fuel if we drive sensibly. Using less fuel is both our responsibility. However, better roads and traffic management is the responsibility of the Government.

[watashi75]

Yes, better road and traffic management could save a lot more fuel than attempts by some individuals to drive at economical speed. There is not much difference in fuel consumed whether a person drives at 40 kmph or 80 kmph provided he is in the proper gear. But, say, if we just had separated lanes for different speeds there would be less braking and acceleration, less accidents and even less road rage, all in addition to fuel economy!

[SS-Traveller]

Originally Posted by teknophobia

The most economical speed on a diesel car is achieved at the highest gear with just enough pressure on the pedal to supply fuel to the engine. Diesel cars usually have the low end torque to keep the car moving at idle rpms

But isn't that true of all cars, petrol or diesel? Isn't best economy for any particular constant speed achieved by "feathering" the accelerator, i.e. depressing it as minimally as required to maintain that speed, and lifting off a little ever so often, to utilise the car's momentum without burning fuel in the engine?

[Ardy]

Yesterday, I was watching Overdrive on CNBC and there was a report on Audi A8 W12. Accordingly to Overdrive, the best FE that they got was at speeds of 100 - 120 KMPH, which of course was on highway from Bombay to Goa.

[zensure]

I had been driving to Mangalore, thru' Shiradi, Madikeri and even via Shivamogga-Agumbe. Important point to note here is crossing over Western Ghat. On my Swift VXi I had got FE 15.5-16.5 KM/L. Barring the ghat section, it was 110 KM/h speed and RPM showing about 2800 at 5th gear. Occasionally touching 130-140KM/h, RPM showing 3200 or so. The A/C usage was more like 70%-80%. All along, never press the pedal hard.

Does anybody has FE experience going steadily at 80-90 KM/h?

What is the optimal speed for swift petrol?

regds

[teknophobia]

Originally Posted by SS-Traveller

But isn't that true of all cars, petrol or diesel? Isn't best economy for any particular constant speed achieved by "feathering" the accelerator, i.e. depressing it as minimally as required to maintain that speed, and lifting off a little ever so often, to utilise the car's momentum without burning fuel in the engine?

It's especially true for diesels, because of the low end torque, petrol engines need to be revved more.

[clevermax]

Around 2000 rpm in 5th gear seems to be the answer for many (petrol) cars.
The speeds may vary, due to differences in final drive ratio, wheel size...

Overheard from somewhere: For maximum mileage, always think that there's a raw egg placed under the gas pedal. Take care not to break it!

[watashi75]

Originally Posted by SS-Traveller

Isn't best economy for any particular constant speed achieved by "feathering" the accelerator, i.e. depressing it as minimally as required to maintain that speed, and lifting off a little ever so often, to utilise the car's momentum without burning fuel in the engine?

There are some inconclusive debates on this. My view is that best fuel economy is obtained when the engine is performing at best fuel efficiency (i.e. peak torque) and is geared appropriately for that speed. So it could be 3rd gear at 60 kmph or 4th gear at 80 kmph. This is under the assumption that load is constant and engine is not lugging. Of course, at higher speeds the load increases because of aerodynamic resistance and changes the equation.

I think instead of asking about the most economical driving speed a more appropriate question would be how to drive most economically at a particular speed.

Consider this. As we know from high school physics, when a car goes from point A to point B, the work done is same whether the car goes at 40 kmph or 120 kmph. The energy required obviously has to come from fuel burned and hence fuel economy should be same. In real life scenario, this is complicated because of frictional losses, aerodynamic resistance, ECU setting, braking, acceleration etc. But this does not change the basic concept.

For best FE - Max speed 80-85 kmph, 5th gear. Anything above that, aerodynamic drag and rolling resistance of the tyres increases rapidly and severely reducing FE.

[anupmathur]

Originally Posted by watashi75

Consider this. As we know from high school physics, when a car goes from point A to point B, the work done is same whether the car goes at 40 kmph or 120 kmph.

Work done may be the same but the energy required will be very different in each case!
To do the work in one hour, you'll need x energy, but to do the same work in one third the time you'll need 3x energy. That's if the relationship is linear, which is not the case. Energy requirement tends to shoot up disproportionately with the speed required!

[watashi75]

Originally Posted by anupmathur

Work done may be the same but the energy required will be very different in each case!
To do the work in one hour, you'll need x energy, but to do the same work in one third the time you'll need 3x energy.

I think you are talking about "Power".

Work done = Force X distance
Power = Work done / Time

In physics, energy and work done are same and have the same units. Power is the rate at which work is done and is immaterial for calculating energy requirements.