Mpg question
I would still just go to neutral instead of downshifting. Why beats up parts for no reason?
TECH Resident
Joined: Oct 2008
Posts: 801
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From: Anaheim Ca.
I read somewhere that knocking in to neutral and coasting to stop uses a slight amount more gas. To save a small of gas, it said to leave it in gear and use the brakes. It said the pcm cuts the gas this way. Is this true? Hell I know! 
Not sure how that would work...but I pop it in neutral as I pull up to a stop sign or just hold the clutch in.
Let's assume by staying in gear you are maintaining 2000 rpms, and by going to neutral you are at 850 rpms. The injectors would have to be pumping out 2.35 times the fuel to maintain idle, than to maintain 2000 rpms. Just does not seem reasonable.
Registered User
Joined: Feb 2013
Posts: 21
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From: Texas
Although I will say even driving around in a muscle car, I do actually try to get the best mpg as possible.... especially now till I get my usual everyday 4-banger working again.
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As soon as you push the brake pedal in an auto car, the torque converter unlocks and RPMs drop to basically idle level.
This may be the case if the converter was never locked up to begin with, but if you're cruising on the highway with the converter locked up and you let off the throttle to either coast or start braking the converter doesn't automatically unlock. There are certain throttle position and MPH criteria to be met before the converter unlocks.
In an M6, if you're coasting while in gear, under certain conditions (RPM range, usually over a certain RPM, like 1200, and a MAP range, say less than 600kPa), deceleration fuel cut-off (DFCO) will shut off all the injectors, so you're using no fuel.
That being said, you don't need to downshift to 3rd if you're coasting at highway speeds (obviously). You'll probably save more $$$ on engine wear than fuel by not doing that. I usually drop it to 5th and ride that out, or maybe 4th if I'm not on the highway.
That being said, you don't need to downshift to 3rd if you're coasting at highway speeds (obviously). You'll probably save more $$$ on engine wear than fuel by not doing that. I usually drop it to 5th and ride that out, or maybe 4th if I'm not on the highway.
How exactly is the engine remaining running with no fuel?
It's just an air pump at that point. The transmission is keeping it spinning, at whatever rpm the speed of the vehicle and the gear you're in determine, so to make power again all it has to do is turn the injectors back on, whether it's because you hit the gas or take it out of gear.
Registered User
Joined: Feb 2013
Posts: 21
Likes: 0
From: Texas
No it doesn't, where'd you get this wild idea?
This may be the case if the converter was never locked up to begin with, but if you're cruising on the highway with the converter locked up and you let off the throttle to either coast or start braking the converter doesn't automatically unlock. There are certain throttle position and MPH criteria to be met before the converter unlocks.
This may be the case if the converter was never locked up to begin with, but if you're cruising on the highway with the converter locked up and you let off the throttle to either coast or start braking the converter doesn't automatically unlock. There are certain throttle position and MPH criteria to be met before the converter unlocks.
*Happysem: Tap the brake pedal when doing about 100 mph+ in an Auto car and tell me your RPM's dropped to idle level. (Never seen that before.)
If you are doing est. 130-160 mph in any auto trans car (well maybe not any) typically your rpm range is going to be well into the est. 4-5k range in 4th gear respectively (Since Auto Trans.) and don't go below that till the car has slowed significantly. > Unless you got some aftermarket T. Converter that does this feature?
Heck the new Camaro SS 6-speeds are in 5th gear doing 155 mph @ est. 5,300rpm and don't go below 4k Rpm till you switch to 6th gear.
Last edited by brnmw; Mar 18, 2013 at 02:50 PM.
It's just an air pump at that point. The transmission is keeping it spinning, at whatever rpm the speed of the vehicle and the gear you're in determine, so to make power again all it has to do is turn the injectors back on, whether it's because you hit the gas or take it out of gear.
Leave the car in gear until about 1200rpm then press the clutch in and finish the stop. It's worth a full mpg if tuned right. It was for me.