[hoomdo]

does it help FE to refuel with the tank half full rather than when its almost empty?

[Atlblkz06]

Originally Posted by hoomdo

does it help FE to refuel with the tank half full rather than when its almost empty?

If you want to get technical about it, it's the other way around. You minimize weight by not loading up your tank with fuel. Gasoline weighs about 750g/l. Lets assume the average tank is ~60 liters. Between full and half weight, that's a difference of 22 Kg. The actual mileage difference is so small, its not worth measuring/calculating.

That said, don't wait till your tank is nearly empty to fill up. I've heard from reliable sources that the fuel pump depends on gasoline for cooling and running bone dry all the time may lead to expensive repairs. I typically fill my car with 10 gallons(37.8l) when I hit the 1/4 mark.

Also - if you're storing your car for extended periods of time - store it with a FULL tank of gas to avoid any moisture related problems and this also keeps your gas from going bad.

[phamilyman]

What about crawling lanes in traffic/toll booths, like <5 kph? when the VDi crawls minimum at 10kph in first gear only!

No one's saying how to minimize clutch? I typically drive in bigger chunks, not the usual practice of keeping 20cm difference with the car in front?

Makes other drivers get very horn-y atleast up here in north, but I detest clutching

[GTO]

Originally Posted by hoomdo

does it help FE to refuel with the tank half full rather than when its almost empty?

I fill my cars up each weekend, irrespective of how full / empty the tank is. I just can't figure out why so many people drive around with their indicator on E. You never know when an emergency situation may pop up. Plus, I wasn't worried at all during the recent fuel supply problem. The last thing I want to do when driving out of the building is wonder whether my tank will go the distance.

Originally Posted by Atlblkz06

I've heard from reliable sources that the fuel pump depends on gasoline for cooling and running bone dry all the time may lead to expensive repairs.

True. Cooling & well as lubricating. A dry tank is the primary cause of fuel pump failure.

[aka_iitd]

Originally Posted by GTO

True. Cooling & well as lubricating. A dry tank is the primary cause of fuel pump failure.

I agree with GTO without fuel pump can fail easily. Not sure it can be related but if you had used summer air coolers which are more common in northern India the water pumps fails many time if there is no water in cooler. Better to fuel your car before it reaches reserve.

[DieselFan]

Additional weight of fuel is too little compared to the evaporation loss when there is lot of empty space in the tank. So it is a quite well known fact that Full fuel in the tank cars give better mileage than the cars with fuel tanks nearly empty. As the fuel in the tank reduces there is lot of air created above the fuel which then induces higher evaporation of the fuel and resulting in the loss of fuel.

[Guna]

Originally Posted by ImmortalZ

You will get less mileage if you depress the clutch and let the car roll freely compared to just letting the accelerator go. This is because modern cars with electronic fuel injection (even the humble 800) use no fuel at all when they are coasting with the transmission locked to the engine. Press the clutch and the engine will start consuming fuel to keep it running (idle running).

It is also a dangerous practice as you'll lose the advantage of engine breaking in case you need to hit the brakes suddenly.

ImmortalZ
Car should ideally consume less fuel when clutch is dis-engaged, especially if it comes down to idle rpm. When enaged, it obviously need to burn more fuel to propel the car and keep the momentum going.
Yes, you may get less milage if you press the cluth and let the car roll freely very often, simply becasue the power generated by engine is simply wasted, without being transfered to the wheels.

[Atlblkz06]

Originally Posted by Guna

ImmortalZ
Car should ideally consume less fuel when clutch is dis-engaged, especially if it comes down to idle rpm. When enaged, it obviously need to burn more fuel to propel the car and keep the momentum going.
Yes, you may get less milage if you press the cluth and let the car roll freely very often, simply becasue the power generated by engine is simply wasted, without being transfered to the wheels.

I agree. I've seen many posts that say that a car does not use fuel when decelerating. I really don't think this is true.

Selective cylinder deactivation (fuel cutout) has been implemented by a few companies and they tell the whole world that they've done something "for the environment" and make it known that the car has that technology!

This would also require direct injection for optimum performance. The engine does run rich when decelerating but I believe fuel is being consumed - but at a reduced rate. I have no proof to prove either side though.

My car pops quite a bit when decelerating and that clearly tells me that gasoline is being used.

[Torque-ative]

I got 21 kms to the liter in my 06 City ZX Gxi !!! I know pretty darn unbeliveable !!?? Well this is what I did left Delhi before 0700 hrs and kept the revs below 2500 rpm all the way, kept the cruising speed at 80 kmph and went easy on the brakes,,,basic physics just ensured I kept the kinetic energy working as long as it could without all that wastefull braking !!

[ImmortalZ]

Originally Posted by Guna

ImmortalZ
Car should ideally consume less fuel when clutch is dis-engaged, especially if it comes down to idle rpm. When enaged, it obviously need to burn more fuel to propel the car and keep the momentum going.
Yes, you may get less milage if you press the cluth and let the car roll freely very often, simply becasue the power generated by engine is simply wasted, without being transfered to the wheels.

When you have your foot off your accelerator and the clutch, your wheels are driving the engine and the other way around. FI cars cut off fuel supply in this case. You don't need to argue this case because it is a well known fact. Of course, it is your prerogative on what to believe.

Originally Posted by Atlblkz06

I agree. I've seen many posts that say that a car does not use fuel when decelerating. I really don't think this is true.

Selective cylinder deactivation (fuel cutout) has been implemented by a few companies and they tell the whole world that they've done something "for the environment" and make it known that the car has that technology!

This would also require direct injection for optimum performance. The engine does run rich when decelerating but I believe fuel is being consumed - but at a reduced rate. I have no proof to prove either side though.

My car pops quite a bit when decelerating and that clearly tells me that gasoline is being used.

Selective cylinder deactivation is used when your car runs at a lighter load - puttering around in the city for instance. When you put pedal to the metal, all cylinders come alive and you get full power.

I suspect that fuel is injected in certain intervals. But it is very easy to check this. Hook up a OBD2 scanner like the ScanGuageII and see the live fuel consumption readouts. For example, in the Corolla Altis, when you take your feet off the accelerator pedal, the current mileage figure in the trip computer hits 99 KMPL (its way of saying infinity). Press the clutch and it comes down to the idle fuel consumption figure. Point proved I think.

[hoomdo]

Originally Posted by DieselFan

Additional weight of fuel is too little compared to the evaporation loss when there is lot of empty space in the tank. So it is a quite well known fact that Full fuel in the tank cars give better mileage than the cars with fuel tanks nearly empty. As the fuel in the tank reduces there is lot of air created above the fuel which then induces higher evaporation of the fuel and resulting in the loss of fuel.

@ DieselFan, this is exactly what I was looking at cheers:. The extra air in the tank also expands more with warming (than the same volume of liquid fuel). This creates more pressure which pushes more fuel. Considering this alongwith, GTO and Atlblkz06's advise about more fuel being easier on the fuel pump, guess its confirmed that its better to refuel as often as possible.

Can we close on this?

[Atlblkz06]

Originally Posted by ImmortalZ

FI cars cut off fuel supply in this case. You don't need to argue this case because it is a well known fact. Of course, it is your prerogative on what to believe.

Hook up a OBD2 scanner like the ScanGuageII and see the live fuel consumption readouts. For example, in the Corolla Altis, when you take your feet off the accelerator pedal, the current mileage figure in the trip computer hits 99 KMPL (its way of saying infinity). Press the clutch and it comes down to the idle fuel consumption figure. Point proved I think.

I've done that - but no change. Come to think of it, I've only driven performance oriented manual cars and automatics. I guess I simply haven't had a chance to experience this.

I've also read that OBD2 cars do not cut out fuel to keep the CAT in its operating temperature. This may be why american cars dont do this.

Also, I think the injector pulse width would probably the best way to tell if fuel is being used. Now I'm curious to see if my friend's saturn does this. I'll hook up my scanner to his OBD and let you know.

[jat]

The thermal effieciency goes down with decrease in atmospheric temp and capacity of power production goes up with the decrease in atmospheric temp - others factors remaining constant.

The fuel economy is better in winter due to AC being off.

[Guna]

Originally Posted by ImmortalZ

When you have your foot off your accelerator and the clutch, your wheels are driving the engine and the other way around. FI cars cut off fuel supply in this case. You don't need to argue this case because it is a well known fact. Of course, it is your prerogative on what to believe.

I think I mis-read what you were saying. Yes, when youlift off, the fuel supply may be cut.
I dont think I argued anyway, just expressed my views

Which would give better FE on slopes - Coasting in highest possible gears without pressing clutch or brakes or coasting downwards in Neutral ?