So I am coasting up to a red light in my modern manual gearbox car, with my foot off the accelerator. Is it more fuel efficent to be in neutral, with the engine ticking over; or in gear, with the engine at the speed dictated by road speed and the gear I have chosen?
I should say I am not looking to perfect a fuel efficent system of driving, it is just idle curriosity! 
I have the same thoughts as I coast to a red light.
I believe it is best to coast, for the most part, in gear and here is why:
First, the gear works as a kind of brake, so if nothing else that saves you some brake. In a fuel efficiency sense, in my understanding (I am no auto mechanic), the car does not idle as quickly in gear as when it is in neutral, the quicker idling requiring more gas to go to the carb-pistons-etc.
I have gleened this mostly from my sparse knowledge of combustion engines and the sound of the engine when idling and in gear. Give it a listen and see if you think what I’ve written makes sense.
Not quite. How much gas is consumed depends on the load on the engine. When it’s idling the only load is the friction of its own moving parts. When the car is moving and in gear, there is the friction in the rest of the drivetrain and air resistance as well. Even though the motor may not be running as fast at low coasting speeds as it is when idling, it’s still expending more energy and using up more fuel.
Oh that’s a good point.
But imagine the engine were made of perfect rolling spheres… :wally
“Saving some brake” vs “saving some engine” is a pretty easy decision. Brakes are designed to slow down the car, and are most reliable when it comes to braking. We’ve had plenty of discussions on downshifting vs braking, and braking always seems to come out on top.
What’s the amount of time when it becomes more efficient to just turn the motor off? I’d like to know for both a bike and a car if anyone’s got the answer.
You won’t really wear the engine at an abnormal rate either way. The transmission is the part that would see this as stressful.
Yep, and if you keep your vehicle any lenght of time (say 5+ years), you’ll eventually start having to replace original parts. Replacing worn-out brakes is a lot cheaper than having to replace a worn-down transmission or clutch.
The only time to really use compression braking is if you have a particularly heavy vehicle and/or you’re on a particularly long downhill slope. Using steady pressure on your brake for a long time to keep your speed under control might cause your brake fluid to boil and this is a bad thing.
It’s going to vary quite a bit from car to car, but the number I’ve heard the most is about 2 minutes for a typical car. Not sure about bikes.
As for the OP, if you’ve got an old fashioned carburetor type car then the fuel at idle is determined only by the set screw on the side of the carburator. It doesn’t make a hill of beans worth of difference if you are braking or using the engine to slow you down. The amount of gas flowing into the carb will be the same in either case. On a fancy shmancy computer controlled car, it’s not so clear cut. I’m not familiar with how the car manufacturers program their computers, so a lot of this is guesswork, but the computer is going to monitor the idle speed and adjust the flow of gas accordingly. At some point you will use less gas if you use the engine for braking instead of the brake, but at some point I would think that the computer is still going to force some minimum amount of gas into the engine, even if the engine is idling at an acceptable speed without any gas (due to the intertia of the car pushing the engine around). Still, it seems to me that overall you’d have a small fuel savings using the engine to stop the car instead of the brakes.
Note - all of this is looking at the problem only from a fuel efficiency point of view, and ignoring all practical considerations about why you may or may not want to do any of the above.
So on a typical car, say a sedan, how long would you have to be braking to begin to boil your brake fluid?
Now I was taught in truck driving school and found on the road that you (if you are in a truck) should put steady pressure on the brake pedal and use low gears. Keep the pressure on the brakes even when you smell them heating up. It works for trucks.
A truck driver named Jake talking about brakes. 
On a car with a carb or some early types of fuel injection in gear or out, would make no difference to the amount of gas consumed.
However when you get to a modern FI engine management system there will be a world of difference.
I’m gonna use the cars I teach on (cause that’s what I know the best) When the engine is warm, throttle is shut, and rpm are above about 1400-1800, the fuel supply to the engine is cut off. So if let’s say the engine is at 3000 rpm and let off the throttle to coast, no fuel will be delivered until the rpms have dropped below the cut in point.
As a practical matter, the amount of fuel saved in very small, but there is a huge improvement in emissions.
so on a modern car, from a fuel saving point of view, game, set, match to leaving it in gear.
Wow - no fuel! Sometimes you get suprised by an answer. :eek:
I can only finds stats on the boiling points of various brake fluids, but the question is too complicated to give a pat answer. It would depend on the temperature of the day, how heated the brake system already is, the weight of the vehicle, how much water contamination may be in the fluid, and how slowly you have to descend (i.e. if you’re stuck behind brake-riding Grandpa and can’t coast). I know drivers of recreational vehicles have been warned about long periods of downhill braking, but a typical sedan? I dunno.
Not quite true! We have discussed the same issue before. The dominant factor that determines fuel consumption is the engine RPM, not load.
What you describe is 100% true for electric motors, but not for internal combustion engines.
From my experience: Reach a speed of 180-200 km/h and brake as fast as you can until the car stops completely. That’s it! The disks are overheated now (and will warp if you park the car at this point). Brake fluid will start boiling soon.
This strikes me as weird. So when I let off the gas at 5k rpm, there’s no fuel at all going into the combustion chambers and combusting? If not, why is there still exhaust noise?
I always wondered if the EFIs were programmed to stop injecting fuel if the output shaft was driving the input shaft (engine braking). You have any more info on this?
OP: Be sure to take into account that you would be able to coast for a longer distance, however small, if you’re in neutral. Otherwise you’ve got the friction and mass of the output shaft and input shaft (if it’s constant mesh) working against you.
I imagine because, even though there’s no fuel going into the engine, there’s still air going through. So you’re still getting “puffs” of air exiting all the cylinders and entering the exhaust manifold. These create the noise you’re hearing.
As a more extreme example, consider the Jake Brake (tip o’ the hat to both Rick and Jake). When the Jake Brake operates, no fuel flows to the cylinders, and the Jake Brake opens up the exhaust valves when the piston is near TDC, causing a blast of highly compressed air to enter the exhaust manifold. No combustion in this case, but very loud.
As I said there is no fuel going into the engine, there is however air going through the engine, so there is still exhaust, which is nothing more than warm air at that point.
There is no advantage to putting fuel into an engine that is coasting down. If you put fuel into the engine at that point, you will generate emissions. This is considered not good.
The only downside to this process is if you have a really really long downgrade, and it is very cold outside, the coolant can cool off so much during the coast down that the car owner brings his car into the dealer, complaining that the heater stoped working partway down the hill. This happens sometimes to cars coming down the Eisenhower grade in Colorado.