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Does Neutral Save Gas or Waste the Transmission?

During my morning and evening commute, i put the car in neutral going down long hills. It takes the engine down to 750 RPM and saves gas. But does all that shifting from D to N and back ruin the transmission? Will i spend my gas savings on a new tranny?

You are not going to ruin your transmission. An automatic transmission is quite capable of dealing with a few extra shifts from D to N and back without injuring itself. Nor will you save any gas. If you coast in D the computer is smart enough to cut back on the fuel fed to the injectors. There will be no measurable fuel savings; leave it in D.

Steve is absolutely right, no gas savings. One more thing - you are losing some degree of control shifting into neutral, through engine braking, so leave it in D by all means.

Your brakes are wearing out faster, you are NOT SAVING ANY GAS, as mentioned, and what you are doing is illegal in a number of states!

Well you are not going to save gas using neutral. On some cars (mostly newer) it will cost your mileage. On these cars the computer totally shuts off all the fuel when the RPM is over the idle speed so in gear over idle speed you use no fuel at all, while in neutral you use enough to keep the engine running at idle.

Coasting down a three mile grade in my 06 Escape hybrid just this weekend leaving Albuquerque, I was curious as to my own theory of watching the tachometer to answer the question related to the vehicle you are driving. We will never be able to answer your question for you on these posts, only speculation, and here’s my results from this 75 mph grade. NO CHANGE IN RPM…NONE…For me I’ll leave it in D.

I’ll disagree to some extent although the amount of fuel you actually save may be negligible. The occassional use of neutral on a hill would not concern me in regards to transmission longevity but if this were something being done at every stop sign or traffic light then my opinion is that it can be harmful to the transmission.
That barely noticeable bump when the trans is shifted into Drive is actually sledge hammer forces being applied to the clutches. One would not slam the clutch on a manual transmission car so why do it on an automatic?

My car has an Instant MPG feature and I’ve actually checked the MPG scenario a number of times.
Coasting down from 65 MPH in DRIVE shows around 45 MPG
Coasting down from 65 MPH in Overdrive shows 65-70 MPG
Coasting down from 65 MPH in NEUTRAL will vary from 75 MPG to a max at 99 MPG.

Now keep in mind that my car only attains those ridiculous MPG readings only for a second and the figure is constantly fluctuating within a certain range.
The MPG feature is pretty accurate and I’ve done this test dozens of times. The bottom line though is that there is not going to be enough gas savings to even make it part of the equation. The MPG feature figures it at tenths of a MPG and since my car (for example) gets 27.2 MPG on the highway it will get that number no matter if I coast a lot or not.
Maybe if it was accurate enough to get into the hundreths it might be noticeable.