Coasting downhill to save gas



I drive a 2000 Volvo S70 with manual transmission. I understand the advice to coast downhill but should I stay in gear with foot off the gas, or should I put it into neutral?


In many states, it is actually illegal to coast down the hill in neutral. Or, at least it used to be, and it is unlikely they have repealed those laws.

Thus, given those two choices, stay in gear with foot off the gas.

The cartalk brothers, I believe, have recommended downshifting to maintain a reasonable speed without burning up your brakes. Do not let the speed get higher than is safe in hopes of saving a little gas.

Of course, it depends upon how long and how steep the hill is, and how fast you are going.


Coasting downhill in neutral is illegal, but clearly such laws are unenforceable.

On a modern computer-controlled fuel-injected car there will be no significant savings. The computer reduces the fuel flow when coasting to about the same as used when idling. It uses the TPS (throttle position sensor) to decide.

So keep your car in gear at all times. There is no economy to be gained from coasting in neutral and the recommended procedure has a small safety advantage.


The important part is the foot off the gas. Depending on the car, the hill wind and what is on the radio, it may be a little better for mileage one way or the other. The real life difference is so small that it is really not worth trying to figure it out. Don’t worry and don’t do something less safe in an attempt to save a penney.


Practically speaking, you’d have to be coasting downhill for a lot of miles quite often in order for it to make any measurable difference. It’s not worth the effort, in my opinion.


My wife’s car has a computer that can be set to show instantaneous gas mileage. When you are coasting, it maxes out at 99 mpg. While it may not be 100% accurate, it seems to be within an acceptable range when I use it to check gas mileage on a full tank of gas, so my impression is that there is virtually no saving to be realized by putting the car in neutral.


I have a 1997 Mustang with a 5 Speed Manual Transmission. I coast at every opportunity and I get very close to 30 MPG. And yes, I coast with the car out of gear, maily because the car will gain speed down hill if out of gear. If the engine is turning at a lower rpm then some gas savings must be occuring, although it is probably neglible. Those laws about coasting out of gear was for the old cars that did not have synchronizers in the transmission. If the older cars got going too fast they could not be put back in gear to slow down. Add brake failure to that or brake overheating and you have a wreck in the making. Those laws are out dated but I suspect they are probably still on the books.


Apparently she has a car that totally shuts off the fuel when the accelerator is not depressed and the engine is running above idle. At that point mpg is meaningless. Of course it also means that someone either already went up the hill (using extra gas) or more often failed to anticipate traffic and was driving faster than the conditions would permit and had to slow down, maybe even braking dumping energy.


leave it in gear. it’s safer. sometimes a quick punch of the gas can get you out of trouble when a different driver does something lame. if it’s out of gear you can’t do that.


Technically, according to DOT regulations, you are considered coasting when your vehicle is out of gear for more than 100 feet, thus leaving you not in control of the vehicle.
Leave your car in gear…Should anything happen while you are out of gear, you can and will be charged with fault.


We’ve had many posts on this and the replies are always the same; it’s dangerous, illegal in many states, and you’ll save exactly nothing in a fuel-injected car. Learn to drive for better fuel economy by being gentle on takeoff, not stopping at the last moment, drive at moderate speed, etc. Those habits will really save you gas!