I’ve been doing some research on getting better MPG while driving. However, I am seeing a lot of conflicting accounts about this practice. So, let’s assume:
Small (modern) 4 cylinder car with a 5 speed manual transmission.
Situation A:
Going down a long (3 mile) hill, is it better to shift into neutral to save gas, or is it better to leave the car in gear (5th) with foot off of the gas pedal to save gas?
Situation B:
Going down a short (.5 mile) hill, same question.
My gut seems to think that shifting into neutral saves gas, but I have also read that taking the foot off of the pedal but leaving in gear uses no gas. However, when going down a long hill I would think some engine breaking would occur requiring the driver to hit the gas to get up to speed again.
If you leave the car in gear with your foot off the pedal, the engine will brake your descent (this may in fact be desirable - or perhaps absolutely necessary) to control the speed, depending on the gradient.
In general though (i.e. if it’s not a 3-mile hill), you’ll arrive at the bottom at a higher speed if you’re in neutral than you will if you’re in gear, and assuming you don’t want to come to a stop at the bottom, that free speed boost will save you fuel on the nest little bit of your journey.
I expect someone will be along shortly to disagree - I’m afraid this is a subject that the SDMB isn’t very good at.
However (too late for edit) freewheeling is generally disadvised on the basis that you are not so close to being in full control of the vehicle. But that’s not the question here.
As Mangetout says, keeping the car in gear when descending down a long hill with any amount of steep gradient is pretty much always necessary…you may even need to downshift. The other thing with going the neutral route OP is that if the hill is steep enough you are going to fry your brakes controlling your speed going down.
In situation A as you describe, MPG is a secondary concern, IMO.
In gear. I don’t believe a modern engine uses any fuel in that situation (completely off the throttle and accelerating) anyways - the ECU turns off the injectors.
That is what I have read as well. However, when I try this in practice, I always end up needing to get back on the gas to get back to speed because of engine breaking.
Engine braking is a negative on MPG, no question about it, so neutral is the best for hypermiling.
At the same time, it has negatives that, to me, make it not worth the effort. Out of gear you are not always ready to respond instantly, you are putting extra wear on your brakes, extra wear on the clutch, etc.
Whether this true or not, engine braking due to above-idle RPM and a closed throttle will piss away the vehicle’s kinetic energy, resulting in a lower speed at the bottom of the hill than if the vehicle had been in neutral - and the driver will then have to step on the accelerator to get the speed back up.
If the grade is steep enough so that you don’t have to get back on the throttle to maintain speed, engine breaking in gear will save fuel. As has been mentioned: The computer shuts off the injectors during engine braking so you use no fuel at all. If you have an instant MPG readout you will notice it going to infinite MPG (or 0L/100km metric) when this happens.
This is a no brainer. I have used this practice for years on a small commuter car I drive daily.
Assuming you have a manual transmission, shifting into neutral on a downhill will always save gas… and it is not insignificant.
The reasoning is:
A - The engine will act as a brake if left in gear. This is something that is well known and used in practice by many truck drivers going down long steep hills with a heavy load to avoid brake overheating.
B - The RPMs dropping alone signify that the engine is consuming less fuel.
I drive a little 4 cylinder Focus, 5 speed to work and I installed a scantech ODBII display for fuel consumption. With it I have confirmed that my own personal MPG went from 34.x mpg to 42.x using this technique.
Additionally my scantech shows that I use approximately 0.48 GP/H (gallons per hour) when idling which is almost identical to the GP/H when coasting in neutral down a large hill. This makes perfect sense as the RPMs are the same.
Finally, any engine that is running, idling or not IS USING fuel. There is no way around this and to suggest otherwise is kind of silly.
In short, the RPMs coupled with your speed generally tell the story of your fuel consumption.
Anecdotally, my car disagrees with you. There’s a roughly half-mile hill down towards my house, and I have experimented with taking my foot off the gas, and also putting the clutch to the floor (which is functionally the same as shifting to neutral).
The instant MPG readout, which on my car will only display up to a max of 199mpg before showing “----”, typically shows around 150mpg with the clutch to the floor and/or the gear lever in neutral, but “----” (i.e. over 200mpg) if I leave it in gear and just coast down the hill with no gas.
I know some newer hybrids do in fact completely shut off the injectors via ECU but for the vast majority of gas only/combustion engines that is not the case. The bottom line is if the engine is running it is using some form of fuel or energy… unless there is a perpetual motion engine I am not aware of that breaks the basic laws of physics
The MPG gauge I installed on my Yaris shows that the MPG is better when left in gear than in neutral. Yes, it does have the ECU that shuts off the injectors while “coasting” in gear. Anyone can research that at yarisworld.com. Real coasting out of gear doesn’t get as good MPGs. I have tested this many times on I-24 near Monteagle, TN. Same on my Ranger,but it doesn’t seem to shut off the injectors as quickly. Both are 5 speed manuals- the vehicles’ momentums keep the engine turning without the fuel.
I have an aftermarket device (the ScanGauge) on my 1999 Ford E-150 van that displays fuel flow and confirms that when descending a sufficiently steep hill, fuel consumption goes to zero.
So I think it’s reasonable to assume that this is normal even in older vehicles.
Right. And the energy used by the engine can come from the gravitational potential energy of a vehicle on a hill - it need not necessarily come from burning fuel.
Many new cars will cut off the gas when coasting in gear once the engine is fully warmed up. If you put it in neutral, then the engine requires fuel to maintain idle or it would stall out.
But in cases where engine braking is not desired, more fuel is consumed than by freewheeling, because you arrive at the bottom at a lower speed, and must then use more fuel to climb the next hill.
The trick is to shift into neutral as you approach the bottom of the hill, such that you reach the maximum safe speed at the bottom and thus climb the next hill with the least expenditure of fuel.
