About a month ago, I posted a thread about a modest experiment in fuel economizing. It seemed like a good idea at the time: Pick one’s foot up off the gas pedal and coast wherever conditions permit. As I noted, the driving I normally do seemed ideally suited to such a technique.
As I say, that was a month ago. Since then, I’ve faithfully followed the coasting approach, finding every little opportunity there might be to let off the accelerator and use the force of inertia for my auto’s motive power.
So, how’s that been working out for me?
It is with surprised delight (and perhaps a touch of smug self-congratulation) that I report the results: It works! It really works!
I’ve done two mileage checks so far. The first produced a figure of 37.5 miles per gallon. The second came in at 36.79 mpg. I will confess to having slacked off a bit after the first mileage check, and am now driving with renewed dedication to the fine art of extending inertia to its safe limits.
So, how much fuel economy have I achieved? According to this website, a 2006 Scion xB should get overall gas mileage of 28 mpg. Not having done a baseline mileage check before the Great Coasting Experiment began, I can’t be sure, but I believe I’d been getting around 30 mpg previously. So this technique is definitely paying off. At 30 mpg, driving 12,000 miles in a year would require 400 gallons of gas. Up the fuel economy to 37 mpg and the gasoline consumed drops to a smidgen over 324 gallons. At four bucks a gallon, that’s some serious savings right there – $1,296, to be precise. Since the price of petrol seems determined to continue its ascent toward the stratosphere, future savings are looking even better.
And now, having tested my hypothesis and succeeded well beyond my modest expectations, I’m gung-ho to keep on coasting. Is 40 mpg achievable? Let’s find out!
I can achieve really significant increases in MPG by coasting (47 or so MPG normal driving, 70 MPG if I drive very conservatively and coast wherever possible) - I’ve heard a lot of objection to it, some of which I’m sure we’re about to see surface in this thread…
I’ve only heard objections to coasting if its done with the car in neutral. I try something similar around town, easing off in advance when I see a red light, so that by the time I reach the stopping the lights are green and I can gently accelerate back up.
I’m trying to figure out if its cheaper for me to take the bus to Cork next month or drive there and back in Mum’s car at 50mph. That speed was given to me by Dad, who did a quick off the top of his head calculation on petrol costs.
I’ve heard that, but I’ve also heard people extend it to say that even coasting with the clutch depressed (in a car with manual transmission) is dangerous too.
But also, there have been a lot of arguments based on misapplications of general physics - arguing to the effect that if you have to go down a valley and up the other side, there’s no way that coasting with the clutch depressed can be any better than just taking your foot off the accelerator.
That argument is wrong for a whole bunch of reasons, but it’s one that comes up here a fair bit.
I was told that the transmission is lubricated by a mechanical pump when the car is in drive. Coasting for long distances does not lubricate the tranny and will cause overheating and severe damage.
That may or may not be true, but my older car’s manual also stated not to coast in neutral over long distances.
I don’t know if newer front-wheel drives have the same lubrication system.
Not dangerous perhaps, but Dad reckons that even with the pedal fully depressed, there’s still a bit of bite between the clutch plates and it contributes to untimely wear on the car. This is the same man who doesn’t see the point in indicating though, so YMMV, no pun intended.
Cecil had a column on the subject but I can’t seem to find it. Basically with your car in neutral, it’ll take you a second or two longer to respond to a danger that you need to drive out of the way of.
Just do a search for posts by Rick. I’m sure about 50% of his posts for the last year or so are spent repeating himself about why you use more gas coasting in neutral than you do with your foot off the gas in gear.
The short version: When decelerating in gear, the wheels keep the engine turning so the car can cut off all the fuel for that time. When in neutral, the engine has to keep itself running at idle, and that requires fuel.
Just west of Mooresville IN there is a road called Keller Hill. Driving down that road is by perception hilly but mostly level. However in truth there is a considerable difference in elevation between the far west end and the far east end (about a 4 mile road total) and another large average hump in the middle.
When I drive out that way to pick up my kids I will shift out of gear and coast from west of center all the way to the east end of the road. The hills go up and down quite a bit but, if I’m going 60mph at the point I shift out of gear, I’m still going 45mph when I reach the stopsign at the other side a little over 2 miles later.
One of the better hills on this road drops at a steeper angle then the road on the other side rises so if I crest the hill at 50mph in neutral I am going well over 70mph by the bottom and still holding at 55mph cresting the other side.
Other than the wear and tear of shifting an automatic out of gear (I usually shift back into drive at the stop sign) while running I don’t see this as especially dangerous. There are no curves so I don’t have to worry about breaking traction and traffic is sparse so as my speed fluctuates between 45mph and 70mph there is no body in my way or behind me waiting.
I can understand how in a corner, when the tires break, it can be important to be able to increase friction while counter steering to avoid drifting off the road. Really though, how often is this going to happen in normal driving whether coasting in or out of gear or not, and how many normal drivers know enough about advanced defensive driving to be able to correct a fishtail strong enough to cause a serious risk anyway?
A Skoda Roomster. It’s a 1.4 diesel. I should note, however, that my figure is stated in UK gallons, which are larger than American ones. It’s about 58 MP(US)G.
It’s not a pretty car though, by any standard - it looks like this:
(that’s not mine - it’s just a random representative photo)
Yes, but that’s about braking on a downward slope, not driving down it with no intention of stopping at the bottom.
On a fairly gentle downward slope, simply taking your foot off the accelerator and leaving the car in gear means you will slow down. If you depress the clutch and coast down the hill, the engine may be ticking over, but the car can roll more freely.
Here's another thing to try. I call it a "burn" after the method used by NASA in Apollo 13. My definition is to use increased acceleration to take advantage of approaching coastable road. So roadway familiarity is concomitant.
Approaching a rise, speed is increased marginally so inertia can carry the vehicle over the hump and into coasting.
Driving a 4cyl.,manual trans 18mpg pick-up the same route for ten years with constant honing of technique yielded 37mpg. Average speeds were likely road stipulations though variation happens both ways.
By coasting I mean in neutral, foot off the clutch so that wheel bearings are (hopefully) the only drag.
I did baseline it a year or so ago, and was getting around 32 mpg. I keep the car (a late 2006 model) well-maintained, so I doubt my baseline would have changed much if at all in the interim.
