Does this fuel-saving tip actually work?

gasoline
fuel-economy

#1

I recently read an interesting fuel-saving tip that seemed completely counterintuitive. Can anyone confirm or deny that this actually works?



I have a '07 Honda Fit with a 5-speed manual transmission and am always interested in getting the best mileage possible. When driving in town and approaching a red light, for example, I generally put in the clutch, shift into neutral, let the clutch back out and then coast to the red light. My thinking is that this will use up LESS gas than leaving the car in gear, where it would have higher RPM (ie, 800 vs 2000).



The tip I read somewhere was that this will actually net me WORSE fuel economy. This fellow’s logic was that when the car is in gear and moving (above, say, 15 mph) and you take your foot off the gas, the engine burns no fuel to keep running. Instead, he thinks, the moving wheels keep the gears moving, which in turn keep the engine moving…all without using any fuel…until you step on the gas again or reach such a low speed that the fuel injectors have to kick in to keep the engine running. He says that with my method, the engine would have to use gas to idle (that is, coast while not in gear) whereas it would use NONE with his method, thus resulting in (marginally) higher MPG.



I guess I can see the logic in this, but it seems to go against everything I’ve ever thought about the correlation between higher RPM and higher gas consumption. What do you think?! Thanks! -Tyler


#2

The engine uses gas in both cases. I think it is doubtful that running the engine twice as fast, it would use less than half the gas per cycle. Maybe if the computer could actually turn off the injectors it could. If you pay to have your brakes done, any savings is lost.


#3

This very question comes up frequently. Join the latest discussion, still in progress, over here:
http://community.cartalk.com/posts/list/1242810.page.


#4

I think this guy’s logic is pretty flawed. If the engine is not running then you have no power brakes.
If you’re coasting in neutral and the tachometer is sitting at 800 RPMs, etc. then where does he think that 800 RPMs is coming from?

Living out in a rural area allows me to do a bit of testing on runs into town. My Lincoln has an Instant MPG feature on the dashboard Message Center and I’ve tested this (auto transmission) numerous times by watching that digital readout; which is actually pretty accurate.
Here’s the specs (and keep in mind the readout is not fixed; it’s constantly being refigured and varies a bit)

Coasting to a stop in Overdrive. 60-71 MPG
Coasting to a stop in DRIVE. 45-57 MPG
Coasting to a stop in NEUTRAL. 75-99 MPG (99 MPG is the limit of the readout)

Also keep in mind these mileage figures are NOT what the car normally gets. It only attains these figures for a brief second or two and in the case of the latter the 99 figure only occurs as the speed drops from 70 down through about 55. Once below 55 it falls into the 50ish or so range.

So the bottom line is that some fuel will be saved. The only question is if it’s enough to really matter.


#5

It depends on the car. My car is one that does turn off the fuel as long as then engine is turning over faster than idle speed. I believe yours does the same.

It is a little complex to figure out the most fuel efficient under those conditions however. There are several things going on. In general I believe that you will do a little better slowing in gear with the foot off the accelerator. One of the problems of being sure exactly when one or the other is best, is the simple fact that the difference is very small.

It is a little larger savings when you are going down a very long hill. In that case you do want to leave the car in gear for safety reasons as well as fuel.


#6

His injectors will continue to feed the cylinders amount of fuel necessary to idle. If not, the engine would die and the CEL light would come on…actually, ALL the dash warning lights would come on, because you’d lose your power to the systems…and that’s what happens when you lose power to all the systems.


#7

ARGGGG. Sorry but is there no end to people wanting to save $2 a year on fuel when a year ago they could care less. Driving gently without jerky stops and starts and proper maintenance will give you the most comfortable and economical drive. Its that simple.

When you let off of the gas, whether the car is moving or not, the throttle position sensor realizes you are off the gas, and tells the computer, that then commands the injectors to a minimum setting along with other sensor inputs like the idle air control. The computer is controling the pulse width to the injectors based on all these inputs from the sensors regardless of whether the car is moving, stopped, clutch in, or out and in neutral. The computer will take care of it if you let it.


#8

The amount of fuel you use is directly related to the engine speed, no matter whether the transmission is engaged or not. If you are idling at 1500 RPM with the transmission engaged or not, it uses the same amount fuel.


#9

Why doesn’t everybody just drive in a prudent manner?

Engine off to save fuel equals no brakes and no power steering.

Trans in neutral equals no acceleration for a quick burst of speed when needed to avoid the guy who turned the engine off while rolling.

Just drive in a safe defensive manner with no jack rabbit starts or lock-em-up stops, just like they taught (spelling) in drivers ed.


#10

Assuming 16 blocks per mile and 30 mpg when driving and 99 mpg when coasting as our 05 Malibu readout says, won’t read higher, and 4 dollar per gallon gasoline, you will spend 0.83 cents per block when driving normally and 0.25 cents per block when coasting. If you can coast somehow at 198 mpg, it will cost 0.12 cents per block. It’s not enough of a difference to be concerned about.


#11

The answer to your question is Yes. In a modern car equipped with manual transmission, computer-controlled fuel injection will stop injecting fuel while coasting in gear, as the wheels will turn the engine and all of the accessory belts.

I recently presented this same question over at a Consumer Reports forum, and the Consumer Reports staff expert stated:

“We have fuel meters that are tapped into the lines to read actual fuel consumption. Coasting in gear uses no fuel to speak of. Coasting in neutral still requires fuel to keep the engine running. The most efficient way is to come to a stop in gear for as long as possible and coast down hill in gear. [emphasis mine]

Any time you are engine braking (on a hill or approaching a stop), you are using no fuel to speak of.

If you have a Consumer Reports membership, you can view that thread here: http://discussions.consumerreports.org/n/pfx/forum.aspx?msg=4234.1&nav=messages&webtag=cr-38aujohn
Post numbers 2 and 8 were the most informative. (The quote above is from post 8.)

I can’t comment on the degree of relative savings (except that it must be marginal at best), only that the CR expert, with his access to specialized testing equipment, confirmed as true the theory you presented.

All this has been discussed on the concurrent Car Talk thread linked above by SteveF.


#12

No one has advocated, asked about, or otherwise mentioned shutting off the engine while coasting. That is not part of the question at hand. I’d like to think we all recognize that would be dangerous.


#13

The amount of fuel you use is directly related to the engine speed, no matter whether the transmission is engaged or not. If you are idling at 1500 RPM with the transmission engaged or not, it uses the same amount fuel.

That is not true of all cars, including mine and I would guess the OP’s car. Many newer computer controlled cars will shut off the fuel when the engine RPM’s are over idle and the accelerator is not depressed. It will automatically start fuel when the engine RPM goes back to idle range.


#14

Absolutely untrue. Your engine could be turning 1500 RPMs while coasting down in gear with the throttle closed, under full load going up a hill with the throttle wide open, and anything in between. The engine speed per se has very little to do with the amount of fuel being used.


#15

My error, I apparently misread.


#16

Excellent post, Scepticus! Case closed, as far as I’m concerned. I’ve already gotten into the habit of employing this gas-saving technique in my own driving. Thanks!


#17

Actually, I just meant to confirm to the OP that deceleration fuel cutoff (DFCO) is something modern fuel management systems do (having only recently discovered this myself). As for his larger question of “this fuel-saving tip”: awareness of DFCO may help a driver strategize driving techniques to save fuel in the long run in various specific contexts. To what degree, I don’t know. There are a lot of variables.

Consider hypothetical hills A and B (in the middle of the desert, both are long and straight and you can see the entire distance from the top): Hill A is a slight downhill grade of 3 miles where Vehicle X can coast in neutral (idling) and coincidentally settle at an appropriate speed (say, 55mph in a posted 55mph zone) for all of the 3 miles. But if Vehicle X coasts in gear, it slows too much and requires the throttle to be applied marginally for all of the 3 miles. Which uses more fuel for those 3 miles? I think coasting in gear, but I really don’t know for sure. (Once again, yes, coasting in neutral is against the law in many places, although I suspect that’s a holdover from another era).

Hill B is also 3 miles, but is steeper than Hill A, so that Vehicle X, coasting in neutral, settles in at 105mph in a posted 55mph zone, but settles in at 57mph coasting in 4th gear (throttle closed, engine is braking, fuel injectors are off). In this case, I’m fairly sure coasting in gear uses less fuel for all 3 miles, and I’m definitely sure it’s safer.

Let’s all drive Teslas and make this question moot: http://www.teslamotors.com/index.php

Anybody got 100 grand I can borrow? I’ll gladly pay you Tuesday for a Roadster today.


#18

Consider hypothetical hills A and B (in the middle of the desert, both are long and straight and you can see the entire distance from the top): Hill A is a slight downhill grade of 3 miles where Vehicle X can coast in neutral (idling) and coincidentally settle at an appropriate speed (say, 55mph in a posted 55mph zone) for all of the 3 miles. But if Vehicle X coasts in gear, it slows too much and requires the throttle to be applied marginally for all of the 3 miles. Which uses more fuel for those 3 miles? I think coasting in gear, but I really don’t know for sure.

In this situation, coasting in gear will definitely use more fuel. Since the car needs zero engine horsepower to maintain that speed, the engine makes zero horsepower regardless of which mode you are using. If you are in neutral, you make zero horsepower with the engine idling at 500-600 rpm. If you are in gear, the engine makes zero horsepower at 2000 to 2800 rpm (depending on your car’s gearing) burning the same amount of gallons per hour that it would be burning if you revved the engine at 2000 to 2800 rpm in neutral while standing still. DFCO does not come into play because if the throttle is closed, the engine is braking and you slow down.

If instead of coasting in neutral, you coast in fouth or third with the clutch disengaged, you are still coasting but you have full acceleration available just by releasing the clutch, even if you coast with the engine not running, much faster than someone coasting in fifth in gear could downshift in order to get full acceleration.