Manual Trans MPG Question (Really simple)

Simply put, is it better to throw the trans into neutral while coasting on highway / down hills, or leave in gear with no pressure on the gas pedal, in terms of gas milage? and why? Out of habit I always idle whilst coasting.


I tried to search this question but I'm not coming up with any results from this forum . Plus its a rather simple Q anyway.

Actually, the answer is not so simple. Is it worth it to do this? The answer is "It depends".

This question will generate hundreds, if not thousands of varying responses. This has been asked before. Try searching on it.

Personally, I think less fuel is used costing in neutral while idling due to engine drag slowing the car in gear (I know, fuel is cut off on engine over run). But this is debatable, as you will find.

Then, there is shift linkage/clutch (slight) wear and the distraction of shifting in and out of neutral to consider.

Bottom line is that I don't do it as much as I used to, but will if I need to slow slightly. If I need to slow faster (as if coming up to a light) I leave it in gear to take advantage of the engine over run fuel cut-off.

--Nate

Well we know what the theory is and since in practice it makes very little difference let me suggest some theory.

Leaving it in gear above the idle speed (about 1000 rpm) the engine will be infinitely efficient since it will not use any fuel at all zero. The computer cuts off all fuel. It also does not slow the car down as much as a gasoline car because the diesel has not throttle.

However there is some advantage possibilities to using neutral as you may then coast further, assuming you are not using any brakes to slow down) and that may be enough to Make up the difference if the grade is less steep.

Joe_Meehan said:

Well we know what the theory is and since in practice it makes very little difference let me suggest some theory.

Leaving it in gear above the idle speed (about 1000 rpm) the engine will be infinitely efficient since it will not use any fuel at all zero. The computer cuts off all fuel. It also does not slow the car down as much as a gasoline car because the diesel has not throttle.

However there is some advantage possibilities to using neutral as you may then coast further, assuming you are not using any brakes to slow down) and that may be enough to Make up the difference if the grade is less steep.

Click to expand...

Exceedingly well written, thanks for your time

Don't mind me, I'm just mining UpOnBlocks' excellent trove of tech info. I just loves me a chart that is good for a couple of minutes of pondering:



By coincidence, that 0.1 gram/sec at warm idle is roughly equal to 0.1 gallon/hour... which is the fuel flow that ScanGauge reports for my TDI. Pretty nice to be coasting up to a stop sign at 25 mph and look down and see the instantaneous mileage showing 250 mpg.

There is a message hidden within this chart concerning speeds at which it is better to coast in gear, slowing down but using zero fuel... versus coasting in idle, slowing down less but using 0.1 gph. I just haven't worked out exactly what that message is yet.

Eddy

That's right, the answer is "it depends"

--Nate

fair enough, i was just wondering if it was more cut and dry... I didnt realize the drag from the engine was comparable to the fuel consumption of idle. Never did the math i guess.

One question.

Why, if "the computer cuts off all fuel" while coasting/going down hill, do I still have a +200 reading on my EGT gauge?

edgewatertom said:

One question.

Why, if "the computer cuts off all fuel" while coasting/going down hill, do I still have a +200 reading on my EGT gauge?

Click to expand...

Well, that is roughly the temp of the block, head, and coolant. Does your +200 reading change with engine temperature?

Eddy

Not sure.

I'll have to do an experiment...go to the top of a big hill...let the engine cool...start the engine...start down the hill...leave it in gear and watch the EGT and temp guages...I'll report back...it might be a while though.

All the metal in the engine is a HUGE heat sink. It takes a good bit of time for it to cool (and warm up). That is why you still have a (high) temperature reading on the gauge as air is being pumped through the cylinders. Also, the gauge may have a certian reaction time. You would think the gauge would fall quite a bit at first (under coast) because the hot exhaust is not hitting it.

--Nate

Anyone have a scan guage? I have one but I don't have a diesel.
drive at any speed, knowing you will have to stop at a light or stop sign, and in trip mode, select current mpg and hit the reset at a specifc point. come to a stop, read the scan gauge. Try it in gear, and try it in neutral.

I would like to hear the readings.

Also, My S-10 PU reads .4gph stopped, in gear, all warmed up. What does a TDI read?

On the my ScanGauge 2 it shows that the Liters per Hour drops to 0.0 when in gear and rolling to a stop or coasting down the road and .8 LPH when warmed up and idleing.