Me and my friend where talking about manuals and autos far as gas goes. I know a stick will use less gas than a auto but the convo was about dropping an auto in neutral. I told him dropping auto will not or use less gas if your in traffic than leaving it in drive. He told me it may use more gas if left in neutral. So who is right?

Also, where not talking about putting an auto in neutral than reving and shifting to drive and launching, thats a no no.

 

I guess neutral could up gas mileage in theory just cause the revs might be slightly lower. In a manual I can see that being the case. Only way it would work really well would be if you actually let your revs drop noticeably, but then you'd be slowing down so for all practical purposes doing the whole neutral would do nothing, since you'd just end up going slower.

 

just keep it in drive and let nature take its course..

i dont think it makes a bit of a difference.. plus you dont want to keep dropping to neutral with an auto

 

1. On a Manuel Car, putting it in Neutral will use gas becauase the engine needs gas in order for it not to stall. If you leave it in gear, lets say first gear in traffic, you will use only the gas when you put your foot on the gas. Otherwise you use Zero fuel. Thats why a car will stall at low speeds on higher gears. So if you want to drive for economy in a manual you should leave it in gear without pressing the clutch, instead of putting it in neutral and coasting.

2. On an Automatic Trans. the same applies, only it uses more fuel because it always selects the gear for you. The engine is always on the gas pedal.

 

WRONG. You will use gas by leaving it in gear with your foot off the gas. Only when decelerating does the ECU turn off the injectors. Once the rpms come down close to idle, the injectors start firing again to keep the car running.

If only that was true. I'd set my idle to about 4000rpms, and leave my foot off the gas pedal all the way to work. I'd save $200 a month in gas costs.

 

Correct!
Neutral uses less gas than keeping it in gear.

 

In an Auto, maybe. When sitting in traffic, in gear, an AT places a load on the engine. Although it's running slightly lower rpms, it'll burn more fuel. In neutral, there's no load, but higher rpms. The difference in fuel consumption in either case is probably negligible, and dropping into neutral every time you came to a stop for a few seconds would kill an AT rather quickly.

 

WRONG! I was implying slowing down. Look at the last sentence of that 1. post buddy. Second the conversation was talking about traffic conditions. Your post has half truths in it. You would set your idle to 4000 rpms? Why? That makes no sense at all. Go outside, put your car in 1st gear and coast the car, if you don't give it gas, you will eventually stall. Modern engines use no gas when in gear and the gas pedal is not pressed. Neutral does, it uses gas to stop the engine from stalling.

 

Does anyone coast on hills in an automatic? I do it all the time in a manual.

 

doing this is very bad on the clutches in the trans

 

he was being sarcastic about the 4,000 rpm thing if you couldn't pick that up. you are correct, when decelerating in gear the ecu turns off the injectors if the gas pedal is not pressed. the car's inertia will keep the engine spinning. the injectors will turn in again as the engine speed approaches idle but if you don't press the gas or put the car into neutral the car will stall.

SO decelerating instead of netural rolling will use less gas.

 

don't shift your auto into neutral unless you need to push it somewhere. end of topic.

 

Hmm I don't see the point, the amount of gas you'd save would be negligable anyways.

 

i heard dere was a test done on manual cars, and that dropping it into nuetral and just coasting doesn't save you any gas at all, n might even hurt ur gas mileage

 

The ECU has a thing call Decel Fuel Trim which basically cuts injector duty cycle (IDC) under deceleration. I don't know if the stock ECU's DFT uses the Vehicle Speed Sensor (VSS) to turn it on and off, but it probably does. So if you're not decelerating then your IDC will remain normal (until you stall if the car isn't getting enough fuel). As Andy stated it's unecessary for there to be fuel or spark present during decel because the engine's inertia and the spinning of the wheels will keep the engine going until accelaration.

Viciouz, while the information is likely correct I think you'd be better off providing sources with more credibility than "my cousin's sister's friend's boyfriend said he saw this test on tv once" if you know what I mean.

stfuad, the amount of fuel you save adds up. If you were to cold start your car several times and then go drive around till empty you'd get considerably less miles to the tank than if you started it once and drive till empty. Why? Because the ECU dumps extra fuel on startup (especially at cold temps) to ensure ignition from a cold start.