Downshifting to slow down, saves gas?

[mabrodis]

So I'm driving yesterday and usually use the gears to slowdown a bit, not excessively but some. I have been trying to drive grandma-ish to save gas, and worked last tank ~15mpg, which is very good for me. I was coasting along, downshifted which ofcourse revved up the engine some. Then I started thinking if that was actually taking more gas. With the engine being pushed faster because of the transmission it would create (atleast I would think so) more vacuum, and if more air gets into the cylinders, then the fuel system would HAVE to put in more gas wouldn't it? If not, then it would be in a very lean condition, which is bad, granted not in that lean condition for terribly long and not on the gas (not powering the engine other than a idle circuit), but still seems like maybe the computer would actually put in fuel to compensate for the lean condition because of the revving engine.

Anybody know forsure?

It seems like if more air got in then it would have to add more gas, but maybe extra air doesn't get it, just because the engine is being dragged faster no more air can get through the throttle body?

If more air could get in, then the computer would add gas (I would think) and therefore downshifting like that would actually be worse for gas mileage than just going into neutral.

Just curious...

Thanks,
Mark Brodis

 

[Gumby]

You're downshifting an automatic? To save gas?
Use your brakes. They are cheap and easy to change. Way more so than transmission clutches.

 

[Pskhaat]

Yep, would agree that unless you actually need the braking of the engine/gearing, use the brake pads up. Those are consumable items, tranny ain't.

Now to answer your question, I do believe you'd use more fuel doing this. For a gasser, I would tend to agree that the computer would keep a non-lean condition and introduce more fuel (this also works the PCV quite a bit). For a diesel on the other hand I feel this would save fuel.

 

[the shed guy]

to save fuel, maybe shifting into [glow=green,6,900]N[/glow] and coast, using up those cheap brake pads might be worth trying?

 

[RavenTai]

on my 81 Nissan (early Bosch FI system) if the TPS shows closed and the engine is above a certain rpm it will cut off the fuel, I have seen this in other FI systems also, I would assume the cruiser is the same (but you know what assume does)

if so you are saved from some of the excessive fuel use, with the TB closed very little air is getting in anyway no matter what RPM so even with the fuel on at lower RPM's you are not using much

as for weather you should do it or not for other than fuel reasons there was a good long thread about compression braking w/ automatics not to long ago with more educated posters on the subject than I, good read if you can find it

 

[mabrodis]

[quote author=Gumby link=board=2;threadid=16245;start=msg155602#msg155602 date=1084401641]
You're downshifting an automatic? To save gas?
Use your brakes. They are cheap and easy to change. Way more so than transmission clutches.
[/quote]
Ok, so I wrote that kinda wrong, I was doing more manual shifting to save gas, coasting for miles at a time, etc. Then downshifting because I was lazy and like the sound of the exhaust..

Obviously brakes are easier to change than anything tranny related (heck easier than even pulling the driveshaft to get TO the tranny)...does shifting the tranny more directly affect tranny clutch life? I'm curious because everyone I've known that has gotten alot of miles on a automatic (over 200K miles) has been very active at shifting manually and downshifting manually at times, probably not excessively but definetly not just leaving it in &#039'.

Later..
Mark Brodis

 

[semlin]

don't know but I agree with you that tranny gears are there to be used and manual shifting shouldn't hurt unless you overheat or overrev them. Can't say that I will engine break in town but I will switch off the overdrive on a steep downhill. I assume that's why they gave me the switch.

 

[e9999]

yup, would be smart if the computer would turn off the gas when coasting downhill. In fact, IIRC some Diesel engines do just that. Not sure what our Toys do, though. I would imagine that they would have thought of it too. I believe that the EPA city etc tests on the dynos do have some significant deceleration periods, so you would think that would show and the manufs would jump at the chance...
would like to know for sure, been wondering about this for a while too...
E

 

[Junk]

even if driving a stick, it will not save gas but rather eat up the gas. when you downshift, you drive up the revs and beat on the engine. saves breaks helps you pretend to be cool etc. don't do it with an auto.

 

[RavenTai]

I checked the manual, from this I infer that the Cruiser does cut fuel if the Throttle is closed (TPS showing closed) and engine is above 1200 RPM, I don't see how this would save gas over normal braking, but it answers one of your questions


"FUEL CUT RPM
INSPECTION
1. WARM UP ENGINE
Allow the engine to warm up to normal operating temperature.
2. CONNECT LEXUS HAND–HELD TESTER OR OBD II
SCAN TOOL
(a) Remove the fuse cover on the instrument panel.
(b) Connect the LEXUS hand–held tester or OBD II scan tool
to the DLC3.
(c) Please refer to the LEXUS hand–held tester or OBD II
scan tool operator’s manual for further details.
3. INSPECT FUEL CUTOFF RPM
(a) Increase the engine speed to at least 3,500 rpm.
(b) Use a sound scope to check for injector operating noise.
(c) Check that when the throttle lever is released, injector operation
noise stops momentarily and then resumes.
HINT:
Measure with the A/C OFF.
Fuel return rpm: 1,200 rpm
4. DISCONNECT LEXUS HAND–HELD TESTER OR OBD
II SCAN TOOL"