Driving fuel efficiency questions (long)

Factual Questions
1

In the light of rocketing fuel prices, my concerns for the planet and more importantly (;)) my wallet, and my inability to do without my vehicle to get around, I have become very interested in driving to save fuel.

There was a little thing on the radio recently about hypermiling, but this stuff seems more suited to automatics with cruise control. Since I drive a bog-standard British manual with no cruise control, I turned to the British government’s lame-ass eco-driving tips, that have at least started me changing up a gear before I get to 2,500 revs, and not exceeding 2,500 revs in 5th gear (which is about 60 mph) when I can help it. It seems to be working: my 50 liter tank usually takes me 450 miles, but since I’ve been accelerating slower I’ve already got 460 miles from one tank and will get about another 10 (and I’ve only been doing this for half a tank too - and my ex has been practicing to drive on the car, which means it’s been revved to shit on that tank too).

But there are a few things I’d like definitive answers for.

1. If moving at speed, does the car in neutral use more or less gas than the car decelerating?

If I’m stationary and put the car into neutral, the tickover engine speed is 750 rpm.

If I’m cruising at 50mph and slip the car into neutral, it does 1,000 rmp (presumably as a result of the computer noting the road speed and pre-empting a change into a lower gear).

If I’m cruising at 50mph in 5th gear, the engine is doing 2,000 rpm. If I take my foot off the gas, it’s still doing 2,000 revs, which decreases proportionally with my slowly reducing road speed.

Several dopers have opined in the past that modern fuel injection systems actually put less gas through the engine when you take your foot off the gas than they do at tickover - but presumably there must still be some fuel. I’m guessing this is because the wheels keep the engine rotating, wheras at idle, more fuel is required to keep the engine rotating. So, is this true, and can anyone point me to some evidence that it is indeed so?

2. If I accelerate fast, does it use significantly more gas than accelerating to the same speed slowly?

Scenario #1: I am stationary and I want to be doing 60 mph. I slam my foot down and I get to that speed after 10 seconds, then cruise for a further 10 seconds.

Scenario #2: I take 20 seconds to reach 60 mph.

I have seen the assertion that scenario #1 is more efficient than scenario #2 because you are at cruising speed faster, but it seems counterintuitive. Does slamming the foot down on the accelerator increase the amount of gas going into the engine by multiples of a gentler accelerator position? Eco-driving and hypermiling definitely think so. Are there any figures available?

3. Does turning my car off then on again waste gas?

So I’m stopped at lights. I turn my engine off completely to save gas, then restart when traffic begins to move again.

I’ve heard that starting it up again will likely waste more gas than I saved with the engine off.

Does it in fact use a lot of gas to get a modern engine started? If it does, is there a cutoff point were I could estimate that the amount of gas wasted by engine restart would be less than the savings made by leaving it on tickover (see point 1!). (I drove a 1980s VW Passat that had this as a shortcut feature on the steering column, but I’ve never seen it since.)
Lots of questions, but they’ve been bugging me for a while, and in particular when this subject comes up in pub conversations, everyone seems to have a vehemently-held opinion, but no evidence to back it up.

2

For #1, I would think it depends on the fuel injection and computer setup of your particular vehicle. One thing is for sure, you will roll farther in Neutural than you will in gear with your foot off the pedal.

3
  1. Maybe, but putting your car in Neutral while driving is a safety hazard, and shouldn’t be done in the name of saving a few drops of fuel.
  2. Yes, accelerating fast wastes gas. When you accelerate fast, a lot of fuel is dumped into the cylinders, resulting in unburned (wasted) gas. It also increases air pollution.
    3)Depends. Hybrids do this trick to save gas. But, if you are only stopped for a few seconds, it’s probably a net loss.
4

I should have said - let’s not get into the safety aspects. I am aware that coasting in neutral isn’t very safe - let’s leave it hypothetical.

I’m looking for a few cites too if possible - if the answer to #1 is only “yes” for specific vehicles, which are the vehicles and are there nice graphs to show it?

5

Wait, you’re saying shifting out of gear while driving is a safety hazard? I could perhaps conceive of how that might be a problem in an automatic, but how do you avoid it in a standard transmission car? You have to immediately shift into another gear? That would be alien to virtually every driver [of manual transmissions] I know.

6

One aspect you didn’t mention was tire pressure. In general, the more air in your tires (within the pressure limits of the tire), the less rolling resistance and the better mileage you’ll get.

J.

7

Yeah, same question. It’s faster to shift into the correct gear from neutral than from the incorrect gear.

8

I think the argument is that you can’t accelerate suddenly if needed, plus you’re free-wheeling and will accelerate faster downhill. Both of these arguments seem shaky. Braking is usually how accidents are avoided, and if acceleration is so important then we should all be driving sports cars.

There are several big hills on rural highways around here that I go down in neutral, I can go almost a mile without touching the accelerator. The trip computer on my Mazdsapeed3 says I’m getting 99.9 MPG while drifting down a hill, but of course that’s all the digits it will hold. During normal driving the lighter you can press the accelerator and still maintain speed, the better MPG you will get. Act like there’s a raw egg on the pedal and if you press too hard you’ll break it.

9

But even beyond that, I always shift into neutral as I’m braking. I certainly don’t row down through the gears to keep the RPM in the “proper” range as I’m slowing down!

10

I’ve heard the rule of thumb that anything less than 60-90 seconds at a low idle uses less gas than stopping and restarting the engine. At a traffic signal it probably makes more sense to leave the engine running, while at a railroad crossing or in a slow drive-through lane, you’ll actually save gas if you stop and restart.

11

Or you can do like I do and drive on empty for like 3 days, babying the fuck out of the throttle to save gas until payday (today).

12

Do you mean that you’ve got the brake applied throughout this time, running the risk of overheating?

13

It always saves gas to turn the engine off. No more fuel is used to start it than you would save if you shut it down. Some cars are being designed to do just that: shut the engine off when at a stop, like a golf cart.

If you are performing the task manually, then it is crazy to have a low threshold of a few seconds. It makes sense economically in all cases, but there are other factors, because sometimes being ‘stopped’ means we are just creeping up slowly, even at intersections.

14

OK, some differing opinions creeping in - interesting - but no cites so far!

Cerowyn, the way we’re taught to drive is that you go through neutral for like half a second while you get into the next gear. You’re never meant to be out of gear for long while you’re moving. Coasting for several seconds with the engine totally disengaged and the clutch released is frowned upon.

15

No, that would sort of defeat the purpose of being in neutral. Just hills, not mountains, I don’t go over 60 MPH or so.

16

I’d like a cite for this. I regularly drop my car into neutral for some down hill stretches on my commute, and I never feel less safe or in control of my vehicle when I do it. How is it less safe?

17

No problem:
http://www.cartalk.com/content/columns/Archive/1996/September/05.html

18

Unless you’re going down a 15% grade mountain this isn’t much of a concern. If your speed goes up too much, brake a little, rev-match, and shift back into gear. I am familiar with the places I roll in neutral anyway, I know I won’t pick up too much speed.

You wouldn’t normally be cruising at a medium/high RPM, so to get enough power for a drastic maneuver you’d need to downshift anyway. Is it going to take any longer to go from 5th to 3rd than it does to go from neutural to 3rd?

Maybe, if your car is prone to stalling… It’s really not road-worthy if it stalls.
I wouldn’t necessarily condone rolling in neutral if there are many other cars around, but if you’re familiar with the road and can stay within the speed limit without riding your brakes I think it’s a great way to save on gas.

19

That’s talking about an automatic transmission. What about a manual?

20

Here’s my take on coasting: Like so many other activities, it’s a marginally dangerous thing to do. 99 times out of 100, it won’t cause any problems, but it’s more dangerous than staying in gear.

I was coasting in Neutral (in my 5-speed manual Mustang) down a long (6 mile) road in Arizona, on the way to Flagstaff. My car stalled. It had never stalled before, and went the rest of the trip without stalling. Turns out I had a bad fuel pump. Anyway, just because it doesn’t happen very often, that doesn’t mean that problems never happen.

Better to be safe than sorry, as they say.