Which method would get the higher MPG??

[drewjp]

This one has been bugging me for awhile.

In an automatic newer car, is it more efficient to shift in neutral and coast to a stop light, or just leave it in drive and slowly brake?

Note that this is for a tranny that shifts into true neutral(drops the RPM back to idle) vs. my mom's car(that doesn't lower RPM when put in neutral).

 

[JHZR2]

if it drops back to idle, then Id guess that it would be indifferent.

In the AT cars that Ive driven, it has always been more efficient to coast in neutral, whether stopping or continuing (per the scangauge). In MT vehicles, youi stay in gear if stopping (for fuel shutoff and compression braking assitance), and go into neutral if youre continuing on.

JMH

 

[greencrew]

Originally Posted By: drewjp
In an automatic newer car, is it more efficient to shift in neutral and coast to a stop light, or just leave it in drive and slowly brake?


That is similar to what a coast clutch does in modern transmissions to get better fuel economy. When the drive takes their foot off of the gas and coasts to a stop, the coast clutch disengages the engine from the trans. It's very similar to shifting into neutral. If you have a computer in your vehicle that shows instant and recent mpg you know that coasting to a stop is one of the quickest ways to increase gas mileage. I think shifting into neutral would be more efficient.

 

[wcbcruzer]

I think some vehicles cut the fuel when decelerating. So if you have an auto tranny that keeps the torque converter locked up, you might actually not burn any fuel while coasting to a stop in D vs. idling.

 

[drewjp]

Originally Posted By: wcbcruzer
I think some vehicles cut the fuel when decelerating. So if you have an auto tranny that keeps the torque converter locked up, you might actually not burn any fuel while coasting to a stop in D vs. idling.


That's what I was wondering actually. I'm not sure what my TC is doing during coasting, but there is quite a bit of engine braking in this car (Mazda Protege 1.8L)

Is there a way to tell?

 

[-Clayton-]

In every car I've ever driven the TC clutch disengages when you take your foot off the pedal.

 

[greencrew]

Originally Posted By: -Clayton-
In every car I've ever driven the TC clutch disengages when you take your foot off the pedal.


That tells me you are young and only drove late model cars with electronic transmissions. As I recall, I think my first was an 92 Bonneville.

 

[chevrofreak]

Originally Posted By: wcbcruzer
I think some vehicles cut the fuel when decelerating. So if you have an auto tranny that keeps the torque converter locked up, you might actually not burn any fuel while coasting to a stop in D vs. idling.


My 96 crown vic seems to cut fuel when decelerating rapidly. Sure sounds funny when it does it because I have side exit pipes with no mufflers. Sounds like it's gurgling.

 

[-Clayton-]

Originally Posted By: greencrew
Originally Posted By: -Clayton-
In every car I've ever driven the TC clutch disengages when you take your foot off the pedal.


That tells me you are young and only drove late model cars with electronic transmissions. As I recall, I think my first was an 92 Bonneville.


92 Bonneville's had electronically controlled transmissions, I think it was the first year for those cars. And they did it too.

 

[Spector]

But then my AWD manual with 6 spd tranny says not to coast in neutral at all.

 

[Eddie]

Letting off the accelerator shuts off the fuel injection. When idling (Coasting in nutral)the injection system is supplying fuel. You will probably see no measurable difference in fuel consumption however.

 

[BearZDefect]

Originally Posted By: -Clayton-
In every car I've ever driven the TC clutch disengages when you take your foot off the pedal.
I've never owned a Toyota but those that I've driven all disengaged this way.

In my 97 i30 (Nissan), however, if it is in top gear with the TCC locked when you let off the accelerator it will keep the TCC locked until you press the brake or the vehicle speed drops below about 36mph.

I am sure some other automatics do this too, perhaps some Fords. The only downside is when the engine mounts get old you can feel the engine flop back and forth getting on and off the gas with TCC locked, just like with a manual transmission.

Modern cars with electronic throttles are another story. They don't even close the throttle right when you let off the pedal, they seem to gradually close it to smooth the transition and perhaps do something for emissions. The 2008 Focus MT I test drove was very much like this, to the point that it felt like an automatic, and I had little real control of the throttle and engine braking.

 

[saaber1]

Results of a real world test with an 07' toyota rav4 rental car w/ auto tranny:

300 miles driven with shifting into neutral whenever humanly possible (highway and town) probably 700-1000 shifts into neutral! This was using a "pulse and glide" technique like the prius.

300 miles driven normally.

The shifting to neutral trip resulted in a 4mpg gain but was the biggest pain in the you know what to shift that many times and to think that hard about driving. I also believe I took off at least one year of life on that tranny by shifting that many times.

Conclusion: Minor MPG gain is not worth it at all if it is your own car due to increased wear and even if there was zero extra wear it is not worth it because it is such a pain.

 

[Gary Allan]

The PCM either maintains or shuts off fuel to the injectors based on conditions and a "fuel strategy". If you're cold, it probably will not shut them off.

I'm pretty sure that contemporary transmissions have over running clutches (sprag or roller) for D(DRIVE) 1st gear (as opposed to 1st selected). This is why your engine retreats to idle below a certain speed.

My tc appears to stay locked when coasting.

 

[BuickGN]

When you think the car is essentially neutraling when you let off the gas, it's probably upshifting to it's highest gear. Take it up to 80mph on the freeway and let off the gas. I bet it won't go down to idle.

Some models don't have engine braking in the lower gears but if you manually shift to that gear it applies appropriate clutches to give you engine braking. No automatic that I'm aware of does not have engine braking if manually shifted.

Torque convertor does not have to be locked to give braking. There will be an rpm drop over a locked convertor when coasting but it will still bring the engine rpms up from idle assuming you're going fast enough and in the right gear.

Coasting in gear with rpms above idle rpm will undoubtedly give the best mpg. If the rpms aren't high enough above idle, some fuel will still be injected but otherwise nothing is being used. If you are using absolutely no fuel, how can you get better mileage than that?

In the GN going down the grapevine, I could get the coolant temp down to near ambient even though it had a 160 degree thermostat.

 

[mstrjon32]

My 1997 Monte Carlo would disengage the TC lockup a few seconds after you took your foot off the pedal. My newer 2006 MC will keep the lockup engaged unless the speed drops too low for it to remain in 4th gear.

 

[eljefino]

Gary and BuickGN nailed it.

Try driving down a hill at ~6 mph in drive, but with the transmission still in 1st gear. Yank the shifter down to manual first. You'll feel engine braking you didn't have before.

Some 4 speed automatics engine brake in manual 3rd (regular D) and some don't. But every one I've seen will in 2nd and 1st and it is in fact a NHTSA requirement. As I understand it even Priuses and other CVTs have manual low settings.

I would be curious to watch a scangauge on an automatic in manual low. Of course forcing these shifts are probably as bad as going to neutral!

 

[BrianWC]

In my 4 speed auto 9-5 (Aisin Warner tranny), as with most other newer trannies, I had thought, yes, coasting in gear would be better IF it were to a stop and you were taking advantage of engine braking. Fuel cuts off and in most cars with instant MPG or scanguage, you'd see "9999" or some variation, showing infinate mpg.

HOWEVER, if you are coasting on a downhill in normal travel and not to a stop, it is more efficient, usually, to shift to neutral as the engaged tranny causes more drag. Any savings you get from the fuel injector cut is negated by the extra fuel you use since you have to reaaply the throttle earlier and/or longer than if you had coasted down the hill in neutral.

Saaber1, as for the mere 4 mpg gain, convert that into a percentage. You might change your mind. For my 2001 9-5 with auto trans, that would be a 19% increase over the EPA combined mpg.

 

[BuickGN]

I've often thought the "grade logic" and braking assist in the TL's PCM is nothing more than a MPG saver. It is nice on hills but usually it downshifts when coasting to a stop keeping the rpms ~1,200. Grade logic works on hills and the braking assist takes some of the load off the brakes but I have a feeling the main reason is to keep it in deceleration fuel cut longer for better mileage.

 

[eljefino]

^ I have to agree with that. Had a mid-90s saturn econobox with automatic. It would not coast to a higher speed on hills or upshift... unless you touched the accellerator. Then suddenly it was freewheeling. I have a hill that leaves an intersection that never needs deliberate accelleration.

In hindsight, wish I put my scan tool on that car to see if it did decel cutoff. I know my stick shift version of same does until it gets down to 1200 RPM.