After hooking up my scangauge, I noticed that my car uses up roughly 20% more fuel idling in drive than it does in neutral, once everything is up to operating temperature. Ultimately, this is an increase of ~0.07 gallons per hour, which means that over 14.3 hours of idling, I'd use up an extra gallon of gas. This is pretty insignificant (about 18 cents extra gas per hour of idling), but I still thought it was interesting how much extra drag the torque converter exerts on the drivetrain at idle.
This isn't correct. How old is the car? On most new cars, when your foot is released from the accelerator e.g. down hill, approaching a junction etc the computer cuts the fuel to the engine to prevent waste. I'm not sure if automatics are the same, i hate the things personally and can't believe people still drive them, but in a manual car you can put the gearbox in neutral and "coast" downhill. People think by doing this it saves fuel as there is no resistance and the car can free wheel. In fact, the engine uses more fuel to idle in neutral so you're better off keeping the car in top gear but not accelerating. I was considering conducting a test to prove this theory as there are many hills around here!
He isn't talking about coasting. He is talking about sitting in Drive going 0 MPH vs sitting in Neutral going 0 MPH. Drive he is saying uses more fuel during idling than Neutral. I would also agree, that sitting there turning the torque converter while going nowhere is going to use more gas as the engine has to work harder to maintain the minimum RPM.
Technically Park or Neutral would be the same if you are wanting to avoid this. Whenever I drive my cars, if I catch a red light, or waiting for more than say 10-15 seconds, I often will flip it into neutral, then just shift back to drive when I can go. If I am going to be sitting still for more than a minute, I just shut it off.
I also travel between Canada and the US frequently, so often when I am on the bridge between the two, if I'm in a long lineup, I will coast with the engine off since gravity will keep me moving along, but be careful. Brakes and power steering can be altered with such practices. I however have been using the technique for some time without mishap and it can save me a lot of gas that I don't use idling in line for 15-45 minutes.
This is one reason that the new start stop motors will be coming in, I think they call them a mild hybrid? I think of it like a gas powered golf cart that when you stop your motor will shut off, hit the gas pedal and it starts right up. DTMAce is right about the gas usage while sitting at a light in Drive. takes energy to keep from stalling the motor. And yes to Fiat500 most cars I would think are going to the no fuel while coasting motors. My 01 TDI shuts off the fuel while coasting especially down hill.
At VWJunky, they're call just that 'Start/stop' lol.
TBH its fairly obvious that the car will use more fuel in D rather than N.
Most car handbooks even advise shifting into N when sitting in traffic, and its basic concideration for those sitting behind you in traffic who you dazzel when sitting on the brakes. One of my pet hates!
For me I'll gladly take the slightly higher fuel consumption rather than shifting into and out of neutral. The reason being that a tranny rebuild is going to cost a lot more than the extra fuel consumption ever will over the life of the car. Every time the transmission gear selector is moved the transmission shifts, there are a limited number of shifts in every transmission and I'd prefer to use mine on the road rather than sitting at a stop.
Point taken bates, and I can see what you mean. Though honestly I've been doing this with my Z26 since I bought it new and now have over 220,000 miles with no problems with the tranny. However each car is different, as are each driver. Just that it worked for me in my experience, and I use it with the vans as well with no problems thus far. I had to rebuild the tranny on one van when I bought it, but that was due to the previous owner's misuse, rather than a gas saving technique.
We all should do what we can and what is feasible to do to save fuel. It helps in the long run. Do what works for each of you, but always consider other options, and keep an open mind.
DTMAce, agreed, do what works best for you. I didn't mean to imply that I had the right answer for everyone, just that by my way of thinking it was the best answer for my individual situation. I also agree with your point on keeping an open mind, I find that often I have been doing something the same way for years or even decades when someone else points out a different and often better method that I'd never thought of.