For better MPG, shift to neutral or leave in gear?

Factual Questions
41

Engine braking is not minimal.

If you’ve got a moderately sloping hill a few hundreds of yards down, then a similar hill up on the other side, entering the top of the system at 40mph, accelerating under gravity and coasting up the other side, you may actually make it all the way to the top, arriving at, say, 20mph.
Try the same thing in 5th gear with foot completely off the accelerator and you will may not speed up at all on the downslope, and will have to apply the accelerator shortly after you start going uphill again, or you will hardly go up the other side at alll.

There are some very good example hills right near where I live - I might do a video comparison of coasting vs in-gear-no-gas (as I’ve driven them both ways many a time) - the difference is quite surprising - in fact, if you don’t freewheel down hills, you’ll often find yourself needing to depress the accelerator to overcome the engine braking.

42

My experience is significantly to the contrary - maybe the size and mass of car is a factor - I drive a small hatchback.

43

True enough, but at some point (the bottom of the hill) you end up exceeding the posted speed limit. That’ll get you a right round roadside rogering in most states, firing squad in others.

44

I suspect the dominant factor here is simply grade, and that posters from various regions have very different ideas of what constitutes a hill.

45

Actually, they do have a clutch. It’s not the same clutch as a manual car, but they have what is called a “lock-up clutch” or “torque converter clutch”. At higher speeds this clutch inside the torque converter locks up to improve fuel economy and decrease heat. It mechanically locks the torque converter input and output shaft together. The clutch is electronically activated by the computer.

46

I have a manual transmission car, and I used to do this on a large hill in my way to work. If Imhit a certain speed coming over the crest I could then coast nearly two miles to the next stop sign.

But…

Whenever I did this I would soon detect a burning smell. Why?

47

Yes - in fact, there’s one above. In the cases I’m thinking of, the hills are either gentle, or short enough that this isn’t a problem - I live in Southern England - we have lots of hills, but no mountains.

In any case, I think I might reconsider my position on this topic, on the following basis:

The ‘idling’ engine of a modern car is actually far from idle - it’s driving the alternator to charge the battery and supply electrical power to a variety of systems, it’s driving pumps for steering assist and brake servos, if the car is equipped with air conditioning, it’s driving that too.

So as you freewheel down a hill, you’re building up speed that may save you a bit of fuel climbing the next hill, but you’re burning fuel to keep the engine ‘idling’.

If you descent the hill with the engine engaged, but with foot off the gas, the rotation of the engine is driven by gravity as your car descends the hill so (as long as it is indeed true that engine management systems cut off the fuel in this scenario), you do have to expend fuel climbing the next hill, but you saved some fuel on the way down - because all those systems were being driven by gravity, not fuel.

I imagine, therefore, there are probably circumstances where freewheeling down a hill may be a slight loss, a slight gain, or a net wash.
It’s going to depend on a lot of factors, including the normal load on the idling engine, the length and gradient of the hill, the desired state of the car on reaching the bottom of the hill, etc.

48

Did you coast in gear with your foot on the clutch? If so, could be the clutch thrust bearing is wearing out.

49

Even when the clutch is fully disengaged, there’s a small amount of friction remaining.

When the vehicle/gearbox is at a stop and the engine is idling at 600 RPM, the speed differential across the clutch is only (duh…) 600 RPM. That’s not a big deal; you’re not producing much heat in the clutch, so you can do this for a few minutes at a stoplight.

When the gearbox input shaft is spinning at ~2500-3000 RPM (as it would be when cruising in top gear at highway speed), and you push the clutch in, now the engine is idling at 600 RPM, and so the speed differential across the clutch is 1900-2400 RPM, several times what it was at the stoplight. You’re creating a lot more heat in the clutch. You’ll end up with pretty high clutch temps at the end of a two-mile high-speed coastdown.

50

The OP’s question has been well answered in terms of saving fuel, but I want to point out that in real-world situations, it can be very dangerous to coast down grades that are long enough or steep enough that you need to use the brakes for any length of time.

In addition to the fact (mentioned above) that being in neutral doesn’t allow you to accelerate quickly if you need to (e.g., a much faster car coming from behind), you risk overheating your brakes if you apply them, even lightly, for long periods.

Frylock (perhaps revealing a natural bias) referred to this in post 5 as “frying” them, but what actually happens is that they become so hot that the brake fluid boils. When there is gas in the brake lines, the brake pedal will go straight to the floor, and have almost no effect in stopping the car. There is nothing like heading for a sharp turn at high speed and finding you have no brakes. Trust me, I know!

So unless you can coast down the hill in question and stay under the speed limit without using the brakes at all, don’t shift into neutral. The fuel savings (if any) aren’t worth the risk.

ETA:Llama Llogophile’s burning smell could also have been the brakes overheating. You should have them checked.

51

Hey, I’m not Frylock!

:slight_smile:

52

There are some settings related to DFCO that CAN be set improperly on your Scangauge.
It can also ‘lie’ about DFCO if the ‘protocol’ (SG1) or ‘mode’ (SG2) is not set accurately.
You can either visit hypermiling forums or perhaps vehicle-model-specific forums to get help with the right settings for your ride.

53

You can gain more control of fuel economy in some vehicles by forcing neutral.
The last several vehicles I drove, and I drive modern cars, all provide at least some engine braking in the ‘D’ setting on the automatic transmission.
Whether or not the lessened control of braking is a worthwhile trade-off to higher gas mileage is a separate argument, and the result likely varies based on where you’re driving.

54

Mass is a factor, as is gearing.
When I drove a Mercury Grand Marquis, a heavy car with a very tall top gear and a low-revving V8, the car would turn a very low RPM at highway speeds. ISTR 1500 RPM at 55 MPH.
It could get up to dangerous speeds coasting downhill in gear on the W.Va turnpike. I had to force downshifts into 3rd in order to keep it from doing that.
When I drove any of the smaller [1] cars I’ve owned since down that same stretch, I had to apply gas in top gear just to keep the engine braking from slowing me down to speeds that were slower than traffic.

[1] In this instance, smaller means vehicles under 3000 pounds when unloaded, as opposed to the 4000-ish pounds of the Grand Marquis.

55

:smack:

I knew I should have checked before posting.

56

This sounds plausible, if only because I never ride the clutch and never touch the brakes in the situation I described. But what about the oil pump? With the engine at idle it would still function, but does its output change with RPM? While coasting could it not be supplying enough oil somewhere, resulting in the burning smell?

57

My experience is also with a small hatchback. A 5-speed manual. As you and others have speculated, there are several factors to consider such as gearing and topography. In any case, the trade off for maybe gaining a few mpg for that short amount of time doesn’t seem worth the trouble to me. In my experience hypermiling tends to produce annoying and unsafe driving habits and I’d rather spend a tiny bit extra on gas than resort to that type of driving.

58

Yeah, you should see the cost of fuel here in the UK though

59

Yeah, I’m sorry, but a lock-up torque converter is not a “clutch.”

From Wikipedia:

60

Not perpetual motion simple physics.
Try this experiment. Rev your engine to 2,000 RPM. Turn off the key. Listen to the engine. Did it instantaneously stop or did it take a short but measurable amount of time to come to a stop? Why didn’t it I instantaneously stop? There was no fuel. Inertia is the answer. All that rotating mass in the engine takes time to stop.
When you take you foot off the gas assuming certain running parameters are met the ECU turns off the injectors while the engine is coasting down. Of course the ECU turns the injectors back on before the engine stalls but all and I do mean all modern EFI systems turn off injectors on coast down.