One day I was about to coast a large (5 minute or so) downhill in my 2.7 manual 5-speed and just to see what mileage numbers I could get (i have scangauge), I depressed the clutch with the shifter engaged in 5th gear. The RPM's stayed at 2,100 (where I was cruising at) for approx 30 seconds before dropping to idle.
Now, with that being said, when I decelerate first before
depressing the clutch (engine engaged braking) the RPM's immediately decrease to idle.
By the way, I accidentally found that coasting downhill with the tranny engaged for more than a few seconds actually shuts the fuel off to the engine (as opposed to coasting by depressing the clutch and idling).
How do I know that the fuel shut off? The scangauge II
shows real-time temperature (water) and it decreased from 195 to about 168 during the 5 minute coast downhill. It can only decrease like that because there isn't any fuel burn to create heat. In that 5 minute coast I managed to go from a trip mileage of 28.6 mpg to 33 mpg. After I resumed normal driving for approx 1 hour I eventually netted 29.8 mpg for the trip. This was obtained at 70 mph average. At the same time I began the descent I noted the (trip), (current), and (tank) mileage increasing dramatically.
The large downhill that I speak of is in the southbound lanes from the top of the Grapevine down into Castaic, CA. Anybody who lives there should know where I'm talking about.
Also, the water temp needle
never moved from it's original position.
Bottom line: decelerating with engine engaged to tranny gets better mileage than just coasting with the clutch depressed at low rpm idle.