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Fuel Efficiency and Slow Driving?
vile8 writes "With the high gas prices and ongoing gas gouging in my hometown many people are trying to find a reasonable way to save gas. One of the things I've noticed is people driving exceptionally slow, 30mph in 45mph zones, etc. So I had to take a quick look and find out if driving slow is helpful in getting better mileage. I know horsepower increases substantially with wind resistance, but with charts like this one from truckandbarter.com it appears mileage is actually about the same between 27mph and 58mph or so. So I'm curious what all the drivers out there with the cool efficiency computers are getting ... of specific interest would be the hemis with MDS; how do those do with the cylinder shutoff mode at different speeds?" Related: are there any practical hypermiling techniques that you've found for people not ready to purchase a new car, nor give up driving generally?
I have a realtime mileage display and variable cylinder technology in my car, and what I have noticed is that I can easily cruise at 75mph on 3 cylinders and get tremendous mileage in the process. However, when I hit an uphill grade, if I try to maintain 75 the other cylinders kick in and my mileage drops dramatically (to roughly 2/3). But, I have noticed that if I gradually back off on the accelerator while climbing the grade, bleeding down my speed to keep those other 3 cylinders from turning on, I can climb the hill while maintaining my high mileage. I've learned also to accelerate slowly on level and mild up grades (like near the top of the grade) without the other cylinders engaging. Obviously when going downhill I take full advantage and build my speed back up while still getting great mileage. Perhaps something like this is what you are observing? BTW, I don't play these games in heavy or rush hour traffic; I only use these techniques when traffic is light.
I would always just leave the clutch in rather than put it in neutral in case you have to get out of trouble.
I remember once, on the M27 in England, a friend and I were driving to Portsmouth. We'd be put just over a pounds worth of petrol in. Gauge didn't even register empty. It was below that. There's a long downhill bit heading down into the city. He put the clutch in and we coasted several miles at least. (This was in a really old Saab.) Little disconcerting, especially when venturing onto a Motorway in the knowledge that your car may break down is an offense.
You have the wrong feeling. Driving the speed limit is a good idea. Driving far below the speed limit (as the submitter specified people going 30 MPH in a 45 MPH zone) is risky and detrimental to good traffic flow and traffic safety as a whole.
The driver going far below the speed limit is likely to incite someone who is less patient to recklessly try to pass them, and that slow driver is responsible for helping to create that situation. To say otherwise is to tell someone poking a bee hive with a stick is not responsible for the stings they receive - that it's only the bees that should be blamed.
In my eyes, anyone who intentionally drives in ways that are counter to how a traffic system has been designed and implemented is putting the rest of us at risk... that means excessive speeding as well as excessive slow driving.
I've studied traffic a lot over the years and what I do know is that it only takes a few drivers driving in selfish ways to really screw things for everyone else. The guy racing ahead on the right to merge at the last minute tends to be the same guy bitching when someone else cuts them off.
And to say "I'm getting better gas mileage, everyone and everything else be damned" is just more of the kind of thinking that leads to all of us getting screwed.
Coasting may be illegal in most US states, but so is travelling five miles an hour over the speed limit. Fortunately, most cops aren't about to pull me over for either (though I do stick to the limit in school zones, for obvious reasons), so I'm not liable to not coast simply because it's illegal.
My reason for not coasting? From what I understand, when the engine's turning above ~1,000 RPM, the throttle's at "idle" (no pressure on the pedal), and the transmission's in gear, then the fuel injectors shut off. For everyone but the parent poster, that means it's not burning gas, and thus raising the mileage. Whenever I might use the coasting technique, it's probably better to simply leave it in gear, let the injectors shut off, save gas, and save my brakes (without worrying about overheating them, too).
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
Take note that I've got a manual transmission in a '97 Subaru Outback. My verification that the injectors shut off is simply "seat of the pants"... there's a bit of a surge when they kick back in at low RPM's.
This is common on a lot of cars. I drive a 2006 Impreza STI and I can absolutely confirm that the injectors are off if you are engine braking. It's the only time EGT drops all the way to zero, even at idle EGT is still around 700*. My 2003 Mustang Cobra did exactly the same thing with the stock tune, verified in a similar way. I'm pretty sure most cars do this -- at least the manual transmission ones. If you leave the injectors firing at all when engine breaking, it causes popping & gurgling afterfires that sound like an old pickup truck.