How does that waste gas? You have to have a wider throttle opening to accelerate, which SAVES gas as long as the opening is so wide that power mixture enrichment comes into play.
With small throttle openings efficiency is poor, because it is the ACTUAL compression ratio that determines efficiency, NOT the geometric one. One needs a high pressure in cylinder during combustion to get high thermal efficiency.
No it isn't.
When you are coasting in neutral, the suspension is unloaded. This means all the parts are loose and floppy. If you have any worn parts, this looseness can translate into loss of control 'very' quickly.
Read your owners manual and see what it states.
Coming onto a world wide forum and telling folks it's ok to coast in neutral is liable to get someone killed.
I off road a lot and see accidents that happen because the person was coasting in neutral. It is bad news to do, especially in snow or slippery conditions.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06"When you are coasting in neutral, the suspension is unloaded. This means all the parts are loose and floppy. If you have any worn parts, this looseness can translate into loss of control 'very' quickly. "
You indicate that for slipperey conditions and off-roading, this is especially true (particularly so for large tires, I'm sure). Won't argue (much) with that.
For street driving, based on my experience with cars in good and bad repair, front-wheel drive, rear-wheel drive, and 4WD, I gotta call bullshit. Maybe has some creedence when going down a steep hill. Maybe.
Dave
Physics are the same on road as off road.
Just read your owners manual. If it is safe it will state so. I'll bet it will say something like mine does: WARNING: Do not coast in neutral. You might lose control of the vehicle.
One problem with doing it is if you get in the habit and mistakenly think you are saving gas, (most FI engines have a decelerate mode that is way lower than idle for fuel use. That is why they stall with no speedo working) you will end up coasting down that one damn hill with the one off camber curve and that pot hole that you couldn't see in the dark or flat light and.....
Besides coasting down hill is illegal in most places. Cops can tell because the vehicle floats loose. I have had a cop tell me this at the bottom of a short hill I got caught coasting down. Got a lecture instead of a ticket at least.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
Coasting down hills is a different story than popping it into neutral 20 or
30 yards before coming to a stop at a light, which is what I, and I think several others, thought we were talking about.
Ok, agreed. I will repeat myself for the part you missed....
In that case, besides if the road is on a slope so it can be called a downhill, most FI vehicles have a deceleration mode that sends less fuel to the engine than a neutral idle mode does.
In the case of Jeep for sure, if the speedometer pickup isn't sending a signal to the computer, it will 'assume' deceleration mode and the engine will stall if you 'coast' in neutral.
To me this means it is more efficient to decelerate 'in gear' as the owners manual clearly states, not in neutral which is dangerous and actually wastes gas...
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
But the fuel savings I am talking about is the fuel savings in running a shorter period of time before you back off and coast. Even with NO fuel going into engine, because of irreversible thermal losses, an "air compressor" takes energy to turn over. A car in gear and coasting coasts a shorter distance, or will arrive at a given point ahead of the initiation of the coasting point at a higher speed, if the car is taken out of gear instead of coasting in gear.
If it arrives at a higher speed when the light changes, it will not need to accelerate from a lower speed. The higher the speed you coast through a light, the less fuel it takes to regain cruise speed. And, the sooner you back off and coast, with a longer coasting distance, the less fuel you needed to coast to any point.
But the fuel savings I am talking about is the fuel savings in running a shorter period of time before you back off and coast. Even with NO fuel going into engine, because of irreversible thermal losses, an "air compressor" takes energy to turn over. A car in gear and coasting coasts a shorter distance, or will arrive at a given point ahead of the initiation of the coasting point at a higher speed, if the car is taken out of gear instead of coasting in gear.
If it arrives at a higher speed when the light changes, it will not need to accelerate from a lower speed. The higher the speed you coast through a light, the less fuel it takes to regain cruise speed. And, the sooner you back off and coast, with a longer coasting distance, the less fuel you needed to coast to any point.
At least you are still alive to talk about it....
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
What the hell are you babbling about? Do you have some insane idea that you can't coast in a car?
When I would like to coast for a while to reach e.g. a traffic light somewhat further away, I sometimes select a high gear like 4th or 5th for coasting. It keeps the engine engaged a bit over idle (1200-1500?), hopefully cutting out the FI's, while not resulting in quite as much engine braking, giving me a longer (cheaper?) coast....
Martijn
You are confusing....
If you do like the law likes and the owners manual states and decelerate in gear, even in 4th or 3rd, you will save gas. The engine will be in decel mode (lower than idle for gas consumption) in 4th at 600 rpm even while you are still doing 20 mph.... And still considered 'in control' of the vehicle.
Floating along in neutral is just plain dangerous and you are fooling yourself for savings.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
You are doing it the correct way! When you use deceleration mode, you save gas big time.
I only gear down at the end when I am about to 'have to stop.
I also accelerate fairly fast up to speed and get great mileage in both of my Jeeps. My 86 CJ7 gets 23 mpg highway and my 88 Cherokee gets 26 mpg.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
Serious question. How is it dangerous?
Steve B.
When you are in gear 'coasting along' or decelerating the suspension is under a torque load and most things like springs, ball joints, u-joints, etc. have a load bias to one side of the mating surfaces. This makes for a basically stable track.
When you are floating in neutral, the joints are bouncing 'up and down' or side to side in the middle of the joint's mating surfaces hitting one side or the other as the vehicle hits bumps in the road.
Think u-joint. A bad u-joint starts vibrating when you are between accelerating and decelerating, it doesn't bitch when it is pinned to one side during acceleration or deceleration.
If you have marginal parts in there for wear, this can be a sloppy mess that can end up with wheels vibrating side to side or parts just plain breaking because they had that 'up and down' impact happening all the time.
When you hit the brakes in neutral, the vehicle also handles way differently, (it nose dives usually) than if it is in gear under torque load and if it has sloppy joints like mentioned, can lead to loss of control really fast.
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
If your suspension and shocks are so knackered that they rely on drivetrain preload to control the car, your rolling wreck shouldn't be anywhere near the road...
Liability and CYA.
I don't know how much gas the engine uses when coasting down hill; there may well be a 'coast' lean program that kicks in. I do know that a zero-throttle engine produces a lot of drag which severely limits my coasting range. Assuming the hill is not so steep as to require/suggest engine braking, I'm all for clutch-in, or neutral gear, coasting, particularly in town, when a stale green or fresh red light is 30 seconds ahead of me. No snap spins or dead pedestrians yet!
This I'll buy. The diff provides some yaw control that might be handy on ice or dirt roads. On clean asphalt, it's a non issue.
-Greg
If a cop told you he could see that either your suspension is shot all to hell, or he's full of crap. Look at it this way: if you're cruising along and go down a slight incline, if it's just enough that the acceleration of going downhill exactly counters the deceleration caused by rolling resistance, then you're effectively unloading the drive train and coasting. Driving with cruise control on you'll pass through such a point (briefly) between the time cruise cuts in or cuts back out. You'd also feel such a transition every time you let off the gas during regular driving. Hell, you'd get the same problem with each gear shift. If the suspension actually unloaded and "floated" at that point, you'd feel a momentary (slight) loss of control each time. But you do not, because the suspension's load is the weight of the car sitting on it. If you want to know what an unloaded suspension feels like, go over a bump at high speed. When your suspension goes from loaded to unloaded and back again you'll feel it! Or drive any suspension with the bushings all shot to hell. Suspensions have tight tolerances for their parts. Enough slop to notice while driving under any conditions means that you need to get repairs, not that you need to adapt your driving style to compensate. The driver's manuals tell you to stay in gear because if you're in gear and shit happens in front of you, then as soon as you let off the accelerator pedal, before you start mashing the brakes, the car will begin slowing down. It also avoids the nastyness of a dumb driver coasting with the clutch pedal pushed in and in the wrong gear and letting the discs whizz around fast enough to pop. Or in the case of a redundant driver, putting it into neutral and holding down the clutch pedal can let some bearings in the transmission get oil-starved since the splash lube will go away but the back of the tranny will still be spinning. By letting the engine fall to idle RPMs, oil pressure falls, power steering falls, the alternator has to work harder to keep voltage up, coolant pumps more slowly, and so on with all accessories. Engine braking extends the life of your brakes' wearing parts and boosts manifold vacuum, which a lot of cars use for brake boosting. And there's always the danger of getting out of gear and for whatever reason not being able to get back in. There are a lot of pros for keeping the vehicle in-gear at all times and using engine braking, but the avoidance of catastrophic, death-wielding failure isn't among them. Writing in the manual, "Don't coast in neutral because we don't think you're smart enough," probably won't go over as well as making up some excuse about suspension unloading.
Physics is the same on or off road....
Check out your local rules of the road (most are online even) and see what kind of 'non issue' it really is.... It can be an expensive non issue if you live where coasting down a hill is illegal a cop busts you for it...
If coasting was so safe as you mistakenly believe, I wonder whey most places in the world make it illegal to coast down hill?
Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06
Please name one single instance of somebody getting a ticket for coasting. How, pray tell, is a cop going to tell if a car is coasting?
It is illegal because there are people on the road who lack the skills to put the car back in gear to accelerate away from an impending accident. Sounds like you're one of them.
I agree completely.
BTW, once upon a time long ago freewheeling was an option available on some cars. And the overdrive on some cars would freewheel too. I'm told by family members who drove such cars that the freewheeling feature was quickly dropped because drivers were getting into trouble on curves and long downhills. The car would gain speed quickly if engine braking wasn't available. Granted brakes are far more fade resistant and more effective now, but finding your car suddenly at too high a speed is still a risk.
Those drivers who pop the car into neutral on a downhill may save a little gas, but I suspect it could be measured by the teaspoonful. As long as the OP is agonizing over minor amounts of money he would have to consider the added wear and tear on brakes too.
It is a dangerous habit to get into.
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