Shifting Tactics

[TheSSG]

I've been meaning to try out some experiments on shifting tactics, but I've been unable to get around to doing it.

Downshift on hills
The theory goes that by downshifting, one uses less throttle. Throttle position is more important than RPM goes this theory.
So, by going 55 @1900RPM in 4th, I'd get better mpg than going 55@ 1600 RPM in 5th, as I wouldn't be using so much of the throttle...
Kinda makes sense, eases the load.
I read it in an analogy:
Riding your bike up a hill in High Gear vs Low; requires MUCH more energy. Going up a hill in low gear, you're moving MUCH faster, but it's overall MUCH easier...



Coasting vs Downshifting
The theory goes: "Modern" cars' ECUs cut fuel to the engine when you downshift (Clutch engaged).
When one coasts, the car is idling, consuming fuel.
Therefore, when the clutch is engaged the Engine is merely "going through the motions" as opposed to idling, and using gas.

Only thing is, this works on "Modern" cars. I've read, FOR SURE, OBDII and up.
Since most of do NOT have OBII+, I don't think it will work.




So yeah, I'm curious, but too busy w/ school to remember to try these out...

[SVOboy]

If you look at your car's fuel map, you will notice that it is down by rpm versus load. As the rpm increases, even with the same load, the fuel injected also increases. This is because your injectors have to pulse more often to accomodate the increased amount of revolutions. So in the end, you'll end up using more gas by down shifting, because even if the load is less, you've got to go heavier on the throttle to keep it injecting enough to accomodate the rpms. Such is the reason the new corvette has a sixth gear for high speed cruising and can get 29 mpg highway, because it keeps the revs and load low at higher speeds than with a 5th only.

[Matt Timion]

Originally Posted by TheSSG

Only thing is, this works on "Modern" cars. I've read, FOR SURE, OBDII and up.
Since most of do NOT have OBII+, I don't think it will work.

This is the one that I really want to know. In my mind my obd0 car gets better gas mileage when I throw it in nuetral and coast. Perhaps it is simply a obII+ thing. At least I hope it is, b/c breaking this habit will be a tricky one.

[SVOboy]

DrDisco69 told me that no pedal pressure is equal to no fuel injection, which I believe, but also that the engine braking will slow you down quicker and you'll spend more time on the gas/accelerating, and won't have as much cruise. So the idea is to cruise in idle longer than cruising in gear and hopefully using less gas.

[rh77]

I believe the Honda shop manual states that at closed throttle, small pulses of fuel are injected for some reason. I'm thinking of confirming this somehow, but I'm not sure. Maybe I'll tie the fuel injector deactivation to all cylinders and rig a switch into the accelerator pedal that cuts all of them at 0% throttle input. The fuel savings may be minimal, not sure though...

RH77

[SVOboy]

Get a scangauge! You're obdII!!! I forgot, I'd love one, tells you trip mpg, instanteous mpg, the ultimate testing aid, and it's accurate!

[Flatland2D]

I've been thinking about this one a lot lately. I think the idea that the fuel injectors are cut off when the engine is overdriven (coasting with no throttle input) is true. I've been leaving my car in gear when coming to a stop. It'll slow down until about 1000rpm then there's a little kick, like dropping the engine back in gear from being in neutral. That is supposed to be the fuel injectors turning back on to save the car from stalling. My car will still drive forward engaged in any gear without any throttle unput. The higher the gear, the fast it goes at idle (though rpm isn't much different). I'm guessing the engine is just trying to save itself.

So I'd say, coast in gear if you're going to be coming to a stop and use zero gas. This would be when approaching a red light or slowing down for traffic. When you are coasting because you're going slightly downhill and want to resume the same speed you started at, coast in neutral. Otherwise you'd probably waste more gas trying to get back up to speed after each coast in gear. If you're going really downhill and can coast in gear and not slow down, then that would be the best way to go.

[GasSavers_DaX]

I agree, it's a balancing act. The coasting thing doesn't work so well for us with automatics though, as generally when coasting @ zero throttle, the car kicks into the longest gear [in my case, fourth] to keep RPMs as low as possible, and then sequentially downshifts through the gears as speed decreases. I can't wait to install my manual tranny so I can take advantage of injector cut at coast.

[Flatland2D]

Is there any reason why automatics don't do this also? I was thinking they worked the same way. I've noticed in my wife's Civic (automatic) that there's a little blip in the rpm's around 1000 when coasting and coming to a stop.

[SVOboy]

My auto generally runs at around 1500-2000 foot of the gas in gear coasting.