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Old 03-01-2012, 02:59 PM   #1
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RPM Question?

I'm not completely fluent in engine details of this vehicle yet. However to my understanding the higher the rpm you travel at the more gas you burn correct?

I work, drive, and practically live in DC. Stop and go, stop and go, stop and go.

So would it be more practical to drive the car in manual mode( auto with paddle shifting). And cruise through the city and keep it in 3rd maybe 4th?

Or do I just drive regular auto and have it put the car in 5th at low speeds. But I was under the impression that at lower speeds in a higher gear is making the engine work harder than say take for example 35 mphs at like 2500 rpms in 3rd vs 35 mph in 5th at like 1800 rpms?

I'm still learning so excuse my ignorance lol
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Old 03-01-2012, 03:09 PM   #3
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its not just the RPM's, you also have to take the load on the engine into account as well.

so if you keep it in 3rd or 4th, you may be lugging it while going slower (and thus burning more fuel because the engine/drivetrain is not operating at max efficiency)

you're best bet is to just leave it in drive and let the computer do the work. You could shift to neutral when you know you are going to be stopped for a while, but be careful when re-engaging the transmission into gear.

I've seen too many people forget its in neutral, hit the gas, then remember its in neutral and pop it into gear only to launch the car much faster than they originally intended.
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Old 03-01-2012, 10:32 PM   #4
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Since you have an automatic, I think that the easy answer is to leave it in drive.

I have a manual and I think that it's an interesting question. The conventional wisdom on the forum is to shift under 2000 rpms. I think that the real answer is not that simple. Mileage is going to have a lot to do with the weight of your left foot. If you push the pedal to the floor, you are going to go through lots of fuel regardless of the rpm that you shift. If you have a light touch, on the other hand, I don't think that you burn that much more by letting the rpms climb a bit.

For example, I often wait to shift until 3000 rpm. If I'm climbing a hill, I may let even let it run a bit more. That's not to say that I'm flooring it. On the contrary, I am giving it just enough to move with traffic. In switching between this and the shift-at-2000 strategy, I haven't seen any consistent difference in mileage (the amount of time that I spend in stop-and-go traffic has a *huge* effect, compared to this.)

My personal opinion is that it may be better for the engine long term to let the rpms beyond 2000 with some regularity. I think that the cylinder pressures are lower, the oil circulates better, and there is less change of knock than by running at lower rpms where the engine may lug.

I'll throw out the caveat here that I am *not* an expert in this. I would welcome discussion on whether my thinking is either in line, or completely wrong - or somewhere in between.
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Old 03-01-2012, 11:07 PM   #5

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In general, your best fuel economy will happen in the highest possible gear. It takes very little power to cruise at 35 mph, so even at a higher gear at lower RPM, you are not really lugging the engine at say 1500RPM or so. My Alero can go into OD at 38 and maintain that just fine at 1100 RPM.

Under a very light load with the throttle almost closed, the engine is only able to pull in a small amount of air past the throttle. With only a very small amount of air available, it becomes very inefficient thermodynamically at higher RPMs, as very little air gets into the cylinders at any given time, thus very little compression actually happens.

Under such a light load, the engine will be more efficient at lower RPMs, as the same limited amount of air coming through the nearly closed throttle will get spread out over fewer firings, i.e. more air will get into the cylinders for each individual firing, increasing thermodynamic efficiency by allowing for better compression. This is basically the idea behind cylinder deactivation as well. Concentrate the limited amount of air into fewer firings so that each individual firing happens at higher pressures/temperatures.

The transition between lugging and not depends on whether or not more RPMs results in significantly more air flow or not. Basically, if the throttle is the limiting factor for how much air the engine can draw in, it is not lugging. If the throttle is open enough that the engine RPM is the limiting factor for airflow (i.e. the engine is running slow enough that the rate of air it is capable of pumping is less than what could come through the throttle given how open it is) then you are lugging and more RPMs would be better.

The best way to tell if you are lugging or not is whether the engine responds to throttle changes. If you give it a little more gas, and the engine responds, you are not lugging it. If you find yourself pushing down more and more on the gas pedal, and the engine is not responding at all to the increase in throttle, that's an indication the the engine is already maxed out for the given RPM you are running at, so more throttle changes nothing.
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Old 03-04-2012, 01:20 PM   #6
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Just leave it in auto and plan ahead, if the traffic is slowing down or stopping,take your foot off the gas and let your car coast. New cars will coast quite far without losing more that 2 or 3 mph, I see to many people speeding up to a stop sign stopped traffic, it just wastes fuel and is harder on the brakes.

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