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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 spent some time researching this matter after a discussion at work started about it.
Something that I had observed in my car was that my fuel economy increased as my speed increased.
At a cruising speed of 85mph, I get 26mpg. at 80mph, I got 24mpg. And at 65, i got about 20mpg. This testing was done along I-10 between Jacksonville and Los Angeles. There's lots of room to set the cruise control. A test usually consisted of fueling up, then a hard acceleration to the testing speed and setting the cruise control to handle maintaining the speed for the next 300 to 350 miles. Individual tests were spot checked (repeated somewhere else on the drive).
In researching this, it wasn't a matter that my car is "faster", stronger, or just plain cooler. It's a function of the drag of the vehicle and the RPMs the engine is turning.
Most cars make their best fuel economy somewhere between 1800 to 2200 rpm. Ah ha! My car has a 6 speed stick. If I'm in 6th gear it's turning about 2000rpm at 85mph.
I then compared ground speed to engine speed ratios of other cars, partly selected because they were owned by people in the discussion, or because they were fairly common cars. Depending on the vehicle, it's best cruise speed could be anywhere between 45mph to 90mph.
Serious? Seriousness is well above my pay grade.
Make those fuel consumption displays mandatory.
Most cars these days know their consumption - it's one of the first things they look at when they connect the laptop to the engine when you go for a service.
Make the display mandatory, make it large, and put it in a prominent place. It'll do wonders for everybody's fuel consumption.
No sig today...
An American Road & Track issue from many years ago (and I'm damned if I can recall which one) had a long article on the results of some fuel economy studies conducted by BMW.
The findings seemed to show that driving style was more important than overall speed.
The tips, in general, were:
- Keep your speed constant; fluctuations up and down are bad.
- Accelerate to your target speed quickly. Spending time slowly accelerating up to it wastes fuel.
- Be in the highest gear feasible for your engine type and road speed.
- 75% throttle for acceleration, conditions permitting.
- Keep your revs low, and change gears often to keep them low. That said, know your torque curve, and use it; if you have a small 4 cylinder, trying to accelerate at 1000 revs is futile.
Using the holy grail of OSes...
Do like everyone else does, drive about 6 inches behind me at 65 mph.
There are sweet spots for driving which is usually specific to the type of vehicle, the gearing, etc. so, to an extent, I'm sure the faster you go the better MPG you will see. But for my car, Mitsubishi Spyder, they recommend shifting into 6th at about 50mph. So basically my interstate driving is all in the top gear by far. At 70-75mph driving on WV interstate highways I get about 20-21 MPG. If I just drop my speed to 65mph everywhere I go during a tank of gas I can reach 24 MPG. I've consistently seen those results out of at least the last 3 or 4 tanks of gas over the last couple months. If I take a US Route (speed limit 55) for 90 minutes to visit my parents my MPG goes up even more for that period of time because I'm going even slower than my usual 65-75 mph. I don't drive too much slower than the posted speed limit (5mph as I state above) because I don't want to feel like I'm crawling but just dropping 5 mph makes a noticeable difference in the range I can achieve with my tank (17.7 gallons). YMMV.
this nation, under God, shall have a new birth of freedom. -- Lincoln, Gettysburg Address
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.
There are lots of little things you can do to save on gas. Many center around efficient stopping.
For example, if I see a red light coming up, I'll often ease off the gas and coast in rather than maintaining speed and then braking near the light like most people do. In addition to saving gas on the way to the light, if the light turns green before you stop then you've also saved the gas it would have taken to accelerate back up to speed.
This tactic can be quite entertaining if, for example, an impatient bozo in a SUV comes up behind you while you're coasting, honks, pulls around you and speeds ahead only to stop at the light, and then you smoke him as you coast through the light just as it turns green.
In a small, aerodynamic car, speed doesn't matter that much. (In a larger vehicle and especially trucks, with their poor aerodynamics, speeds above 60 do start to affect mileage more strongly.)
But how vigorously you accelerate can make a big difference. In the worst of the gas price spike I made a point of accelerating gently and shifting much earlier than usual, and found my mileage improved by 15%.
Your god may be dead, but mine aren't!
Yeah. There was an article in Readers Digest a few months ago about "hypermiling" or whatever, and it was a case study in "I'm the only person on the road". Incredibly arrogant and self-centered.
No folly is more costly than the folly of intolerant idealism. - Winston Churchill
Oh, for mod points. Most people (well, most men anyway) are competitive, and we like to beat our "high scores". Tachometers show us speed, clocks show us time, but neither of those contributes to efficiency. Adding a fuel economy display gives a better goal to beat.
If you were blocking sigs, you wouldn't have to read this.
I'd say the way people blast off from the green light like their in a Formula 1 Grand Prix* is probably doing a bigger number on fuel economy in city driving more than anything else.
* or not if you were Hamilton yesterday.
"The great thing about multitasking is that several things can go wrong at once." -me
I always thought those people were assholes, and I'd fly into a rant about how dangerous and reckless that behavior was. But they're just trying to save money. People really are basically good after all!
Oh, yeah, it's not easy to pad these out to 120 characters.
This is just another case where people don't realize (or care) that trying to maximize the performance of one part of the system (their commute) ends up diminishing the performance of the overall system.
Only a few people doing this slow driving will result in large numbers of other driver stuck waiting at more lights. Even worse, this kind of slow driving will result in some other drivers driving recklessly trying to get around the slow drivers. It won't take many crashes, injuries, and deaths to completely wipe out any savings made to the economy by a few people driving slowly (if only from traffic backups due to crashes).
Using these kinds of hypermiling techniques are just fine for an individual who doesn't have any regard for how their behavior impacts others.
..is accelerating relatively fast to something like 70mph,
then pull of the engine and roll with no gear until You reach
something like 10mph when You start the engine and repeat.
This is the empirically show best method.
But it will probably irritate other drivers...
Mundus Vult Decipi
Hypermiling isn't even remotely about slow driving. It is about accelerating at an optimal rate, cruising at an optimal rate, and carrying no more speed than necessary to get to the next known stop.
Pay special attention to that last one. Carrying no more speed than necessary to get to the next known stop. A hypermiler's behaviour isn't going to affect anyone. If they were all going to be stuck at the next red light, they were all going to be stuck at the next red light. If they were going to make the light, everyone can cruise at their optimal rate.
A hypermiler's behaviour only impacts how other drivers _think_ they are doing in terms of making good time to their destination. Such other drivers love to do things like see that a light is turning red and then _accelerate_ towards it because they want to be first in line. Or because it just feels good. Or whatever. But they'll be waiting at that exact same red light as everyone else, including the hypermilers.
Posts like yours place the blame here on the hypermilers, but the blame should reside elsewhere.
The throwout bearing is also known as the Jesus bearing to those who wrench on cars. Usually after rebuilding the engine, installing it in the car, and topping up the fluids, you'll notice the Jesus bearing sitting on top of the toolbox.
I have an 2001 Sentra and just inst hooked up a gadget I got from Think Geek (ScanTool, I believe its called) that reads the engine computer through the OBDC2 connector. I can verify that taking my foot off the gas does shut off the injectors if the car is in gear and going fast enough. From the ScanTool manual I infer that this behavior is common, but not universal among cars.
The logic is that the majority of people are going to drive at a certain speed on any given road regardless (the "85th percentile" rule) and the one doofus going significantly slower than this becomes a very unexpected, slow-moving obstacle which requires people to either hit the anchors suddenly, or attempt to swerve around, both of which are clearly unsafe behaviors.
While most cops won't care about this excuse because they want to maintain a ticket quota, many judges will, assuming no other violation and a good attitude, accept the "I was just keeping up with traffic" line as grounds for dismissal or reduction of a citation. There's a reason for this.
I grant you that this study, and some others like it, mention only accidents and do not discuss or even mention fatalities, but the reduction of total accidents when everyone drives at the 85th percentile is a pretty clear fact. If everyone drove slower this probably wouldn't be the case, but since we aren't going to change the rset of humanity's driving patterns, telling people to drive slower than they should is dubious advice.
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