Coasting: Good or Bad?

[floodx]

Alright, to let you know where I stand: I got my first standard car less than a week ago. I've driven a standard vehicle twice before this. I'm also the kind of person who researches like mad in situations like this. So I have a fairly good idea of how a clutch works and a grasp on some basic techniques. I can pull off rev matches pretty good (albeit a bit slowly, but still...). My shift from 1->2 is still a little sloppy, but I can shift between the rest like butter.

My problem is: No where could I find good info on coasting. Apparently it's illegal in some places though? This is where I'm confused. I drive almost strictly in city traffic and a lot of times instead of shifting up when my revs hit 3-4, I'll coast if I know I'll have to slow down anytime soon. Then just rev match in the appropriate gear when I need to accelerate again. And of course I always rev match before taking cornors unless I'm doing 10k around a parking lot.

Does anyone see anything wrong with this? Tips or complete driving strategy revamps welcome. Thanks!

[watkins]

Welcome!

Driving in the city is pretty easy. Rather than rev high, which I dont do in normal driving anyway, I shift early to keep near idle speed and just cruise along. If I have to slow down but not stop, I downshift. Thats the beauty of a manual. You also dont get massive engine braking at low revs, which makes for smoother driving overall.

Of course I hate coasting in any situation other than from idle speed in gear X to a complete or near-complete stop

[theholycow]

I don't drive in the city much. When I do, I generally find that I need to be aggressive and fast, which tends to preclude doing much neutral coasting.

When that's not the case, though, I probably do more coasting than not. There is nothing wrong with coasting. If you enjoy it, do it.

In many states, it is illegal to coast downhill. I'm sure everyone here does plenty of other illegal things while driving, too. It's also illegal to exceed the speed limit (even by 1mph even only for 1 second), not come to a complete stop when making a right turn on red, or to drive more than 15mph when within 1500 feet of a mobile vending business equipped with flashing yellow lights.

I don't advocate breaking the law. If you're going to break the law anyway, I don't see downhill neutral coasting as being any worse than any other driving law you're going to break. It's not dangerous (unless you're a terrible driver and need instant blistering acceleration on tap at any time; in which case you should be driving a 1000hp automatic, or a 1000hp manual in the lowest gear possible). Practically speaking, you won't get caught neutral coasting, unlike almost every other violation which is visible from outside the vehicle.

Why do you want to coast? If your car is aggressive and consistent with its DFCO behavior, you might save more gas leaving it in gear while braking. When you don't intend to slow, neutral coasting is still more efficient (but we're not talking about doubling your fuel economy or anything extreme like that).

[Swimmerboy]

Theholycow just summed it up for you lol. All i can say is that it's not a bad thing to coast.

[1974Alfa5spd]

Welcome floodx! It might help us a bit if we knew what kind of car you're driving.

Although in may be it's illegal to coast downhill, I don't think that's what he's talking about.

Coasting up to a stop sign or a light is probably also illegal, and where that no-coasting law comes in. If you coast up to a light, you do not have complete control over the car, so if say, a fire truck came roaring up behind you and needed to get through, he'd have to execute a dangerous maneuver to get around you (little do the law makers know, it's not that hard to get a manual or a slushbox back into gear. ). I consistently coast up to stop signs. Sometimes I can make it though a line of six cars without touching the clutch, just from the inertia the car has, I can creep forward at about 1/2 MPH.

Usually what I do for stoplights and crowded stop signs is begin braking about 200 feet beforehand, then when the RPMs hit about 1600-1700, I clutch in, and coast, pull the shifter into neutral, and come to a stop. By the time I clutched in, I am about 20-35 feet away from the (stopped) car in front of me, so I am "coasting" very little.

[theholycow]

I have not seen anti-coasting laws that apply anywhere outside of descending a hill. They all treat neutral or declutched as the same. You have to be in gear with the clutch engaged to obey the law. Can't do that when rolling slowly to the stop.

If you need instant "go!" power, you need an automatic. A manual just doesn't offer that. The fire truck will have to wait while you engage the clutch anyway. If it's difficult to get your car into gear then you probably should not coast to that stop, but a modern car that can enter gear quickly is fine.

paul34, your R32-owning friend has observed that DFCO just isn't that dependable. There's plenty of conditions that cancel DFCO. Recent gear change? No DFCO. Recently romped on the throttle? No DFCO. Quick change in speed or RPM? No DFCO. Air conditioning on? DFCO floor is raised by 200-400 RPM.

People think of it so black-and-white because they've never observed how it really works, they just read about it and assumed that it's simple. It's not. It's important to know your car's behavior.

[theholycow]

There are ways to test DFCO, but they're slow and inaccurate for knowing about the few seconds at a time we're talking about. When you think you're DFCOing, turn the ignition off and see if engine braking increases. If it does, you were not DFCOing.

ScanGauge is very slow to update and terrible for training your butt-dyno to detect DFCO. You might DFCO and then two to five seconds later the ScanGauge reports it.

Something that directly measures a fuel injector wire will be much better. I've used a duty cycle meter (actually a digital dwell meter) for the job, worked great and really helped me understand my DFCO conditions and delays.

[watkins]

when the RPMs hit about 1600-1700, I clutch in, and coast, pull the shifter into neutral, and come to a stop.

I wait until 1100-1200. DFCO needs to do its job first.

[1974Alfa5spd]

when the RPMs hit about 1600-1700, I clutch in, and coast, pull the shifter into neutral, and come to a stop.

I wait until 1100-1200. DFCO needs to do its job first.

Well, I don't have DFCO and the car starts bucking when I take it that low. Stupid lack of low-end torque.

[watkins]

There is no reason the car should buck

[AHTOXA]

As far as coasting - there is no rule of thumb because it depends on the situation. I will coast in the city sometimes when staying in gear will provide too much engine braking and shifting to the next is pointless because I will have to go to neutral soon after anyway. This happens when I need to dash between lights sometimes for various reasons or the like. Otherwise I hardly even coast out of gear. I adapt my driving and timing so that I use engine braking as much as I can while not being obnoxious by going too slow for traffic or downshifting every gear all the time. This eliminates coasting time to a minimum for the most part.

[lilgto64]

Not sure that I have seen any laws regarding coasting - or what the hazard would be - if there are such laws they may be aimed at a couple of situations - 1 "free-wheeling" where you use the downgrade to accelerate using gravity - in which case you are exceeding the speed limit - 2 turning off the engine entirely - in which case you are more likely to lose control of the vehicle if you lock the steering wheel or the engine does not start again etc. Both of which are more hazardous with an automatic.

I have seen signs posted indicating areas where engine braking is prohibited - which I believe is aimed at trucks which get rather loud when engine braking - and these signs tends to be posted where there are residential areas immediately adjacent to the roadway.

If it were illegal to coast with the car not in gear - then EVERY time you shift a manual trans you are braking the law - since you will necessarily be in neutral during the shift.

I have been driving a number of different makes and models of cars with manual trans for many years and can tell you for sure that coasting when it makes sense to do so saves gas, and saves wear on the brake pads and rotors. You do have to be more aware of your overall situation - speed, distance, gear selection, etc than someone driving an automatic.

The things I have read about things like needing to react at a stop sign to emergency vehicles or accidentally slipping off the clutch at a light etc don't make any sense to me as they are not really any different than the reaction time or slipping off the brake that an auto trans would face. In fact, unless you are driving your automatic with both feet (one on the gas and the other on the brake - then I can react faster with a manual than you can since I have one foot on the clutch and the other on the gas ready to go - even when not in a level area.

Without coasting you would either be applying the gas or the brake at all times - with an auto or a manual trans - and being the passenger in a car with a driver who is either gas on or brake on and never in between can be quite scary.

The way I often get off the local freeway - provided the I catch the first light - I can make it all the way off the freeway all the way home without touching the brake pedal - by coasting or engine braking but mostly by being aware of what is up ahead and knowing how long each light's cycle is and using cues such as how many cars are stopped either in front of me or at the intersecting street - all without exceeding the speed limit - or compromising my control of the vehicle - or reaction times - etc.

In fact there are times when coasting with a manual allows you to do things such as anticipate that you will need a lower gear and be prepared in ways that an automatic trans will never be able to replicate.

[theholycow]

I agree with the new guy.

Not sure that I have seen any laws regarding coasting - or what the hazard would be - if there are such laws they may be aimed at a couple of situations - 1 "free-wheeling" where you use the downgrade to accelerate using gravity - in which case you are exceeding the speed limit - 2 turning off the engine entirely - in which case you are more likely to lose control of the vehicle if you lock the steering wheel or the engine does not start again etc. Both of which are more hazardous with an automatic.

The laws do exist, and they're vestigial from when brakes and transmissions sucked. There used to be a strong risk that, on a long descent, the brakes would fail and you'd be unable to get into gear. Neither of those things are anywhere near as likely anymore.

[07_Mustang V6]

The steering wheel doesn't lock if you turn the ignition to the 3rd position (one before actual ignition) right? At least I didn't think so.

[theholycow]

The steering wheel doesn't lock until you turn the key all the way to "lock". "Off" is just before "lock" and you can't pull out the key until "lock". The engine will shut off before you reach "off", let alone "lock". You can then turn it back to "On" without the engine restarting (unless you've left it in gear).

Engine-off coasting is not necessarily a great idea, though. You only get to use your power brakes 2 to 4 times before you run out of boost, and you have no power steering unless your car has electric power steering (though at speed, you probably don't need much assist anyway; isn't that what speed-sensitive power steering is all about?).