You'd be doing much, much better to just lay off the gas pedal as you crest the top of the hill and let the truck run down the other side in gear but with no gas. If you put the transmission in neutral then it's a near certainty that you will eventually forget that and rev the engine needlessly as you start to try to accelerate again after you're off the hill, and that might damage the engine. In addition to that, it's just safer all around to have the "engine braking" if it turns out that you need it sometime. You can try it, though. And if you have a tachometer you'll probably see that the engine is doing around 800 - 1000 rpm as you coast downhill, whether you're in gear or not, so that 'savings' is generally mythical. Laying off the gas whenever you can, and planning for what's ahead, will do you lots better over the long run. Coasting up to stop lights and stop signs is also one of the best ways to save -- on both fuel and brakes.

another thing to think about, when you are in gear, you can feel the engine slowing you down a little... which means the cars momentum is "powering" the engine, not the fuel... and when you put your car in nuetral, the fuel is powering the engine. For "proof" of it being better to leave it in gear, go test drive a really non-fuel efficient vehicle with an instant mpg feature and try it. My car (grand prix GXP w/ V8) gets 99mpg in gear going downhill, but only 50-65 when in neutral. Will not harm the engine either way, as the oil and coolant are still being pumped, and the injectors will not spray if they do not need to. Hope this helped. Agree with eveyone's points so far. Only able to give +1 to people so everyone got it. :) The only comment I disagree with is the speed difference. While you do save a little money going slower, its not a lot and not worth it. At slow speeds, and short distances you will not really notice it at all maybe a couple pennies each trip. Long distances, you'll really see the difference, and its usually still not a lot. Personal example: From Grand Forks, ND to San Antonio Tx, its 1,415 miles. Using my impala, going 60mph (rated at 30mpg) would take 23.5 hours and us 47 gals. But driving 80 (27.5 Actual avg) would take 17.5 hours and only 51.5 gal. 4.5 gallons more but 6 hours less. at $3.50 gal- only save $15.75 but cost 6 hours. my time is worth more than 2.5 dollars an hour :) If you dive a really inefficient vehicle, or the change between 60 and 80 mph is a lot on your mpg then figure it out and determine if its worth it. Like i said, i agree it does save some money, but only if your willing to give up the time. Hope all of these ideas from everyone help you, and anyone else looking to save money.

With the electronic controls in modern transmissions you're probably not going to torque it much - but with the electronic controls in a modern fuel injected engine, you're not going to burn any less fuel in neutral than you would leaving it in gear and in neutral you lose that facet of control of the car and engine breaking. The biggest differences will come from driving 60 instead of 80, by starting and stopping moderately instead of with the pedal to the floor, and by combining trips where possible so you're not making repeated trips with a cold engine.

I average 41-42 in mixed driving and can get over 50 on the road if I hold it to around 60. I prefer diesels to hybrids, especially for freeway driving - they pull the hills better. :) The 10% is there in mixed driving -- you're right, it's not there if it's mostly city driving. I can get a 20% increase in exclusively highway driving. Very few of my trips are just around the block with no freeways at all. Even the drive to work is over 20 miles. The decrease in mileage at higher speeds is due to wind resistance. There are conflicting studies on whether gentle or rapid acceleration is ultimately more efficient over all. TTAC, Autoblog, etc. have run articles on it fairly recently - some of the better studies favoring rapid acceleration were done by BMW... go figure. LOL. In my cars gentle seems to work better, but I mean gentle not hypermiling. What hurts me more than anything is outside temperature. I will lose 5 MPG or more when the temp drops below about -15. Forunately that's only a few tanks a year.

no but you will not see never the gain of a manual

Shfting into neutral while going over 30 is`not advised. In our car it showes us our mileage and it does the best to just keep everything steady. If your going down hill, slowly let off on the gas, but if you let off on all the gas, you will burn extra fuel when the hill ends and you gotta accelerate.

i heard that this damages the transmission. i suggest if you have a small engine just work with what it has to offer, that is small engines 1500cc and less dont really burn much gas. if u have a large engine try finding other methods.

All very intersting. The only one that mentioned the true harm wasn't sure. When you put a vehicle that has an automatic transmission in neutral and coast, the pump does not correctly lubricate the transmission. That's why they tow rear wheel drives backwards. To get better gas mileage, when you see the light ahead is red, slow down and just coast up to it instead of waiting 'til you get there and then using the brakes. Also, avoid racing away from the lights. I drive a car that is only listed to get 27 mpg and get up to 33 mpg using this method and driving 55. All things considered, I would probably just be sitting down at the computer writing to AB so I might just as well drive slower and take a little longer. (If you are driving a Prius, the trick is to take your foot off the gas when going downhill and then just barely touch the gas pedal...it's called floating)

what kind of vehicle do you have?

I don't know what the effect would be on an automatic transmission. I've been driving a manual for a very long time, and I get a lot of use out of nuetral. It has had no ill affects on my vehicles and it really raises the gas mileage.

Since you're going to take your foot off the accellerator anyway on a downhill, why go thru the trouble shifting into neutral? When you get to the bottom of the hill you have to shift into drive again and it could be hard on the tranny because an automatic goes thru a first gear at a lower speed, then (automatically) shifts into second gear. Shifting directly into drive at a high speed doesn't allow the tranny to do it's normal job and could be hard on it. Why bother anyway? Just tap the brakes so you keep the speed limit...unless you know there aren't any cops around. Fun! The others mentioned best gas mileage and I say the same. Look ahead for signals and COAST up to them. It will also save wear on your brakes. I wonder why these idiots race up to lights then stomp on their brakes then stomp on the pedal off the line. i start off slow too. Any speed over 55 means the more gas you'll use, so if you want to push the pedal that's your choice. I got some things you screw on your valve stems that keep track of my tire pressure. If it goes below the right pressure they turn red. $5 at Wal-Mart. Bad tire pressure=bad mileage. I do yearly tune-ups, including fuel & air filter. That plus a yearly oil change will improve mileage & also make your car last longer. Some people even change their oil twice a year if they drive a lot. Schedule your errands to do a lot at once instead of one at a time. Even a small car will get lousy mileage if not driven right or maintained right.

It's not necessary to do so. When you are going downhill and don't need to give it gas to maintain speed, the engine is already at idle. Moving it into neutral will not save any more fuel. If by chance you need to accelerate quickly to avoid a collision you would be at a disadvantage if you had to fumble with the stickshift before doing so.