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Lowest RPM for switching gears (manual transmission)?

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Old 02 Dec 2010, 08:58 am
ProDigit's Avatar
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Default Lowest RPM for switching gears (manual transmission)?

I've been playing with a thought, that I try to put into words here.
I've written this thread with the thought of a man who wants to savor his car as long as possible, and drive cautiously. Not meant to be written for racing environments.
Would you think the beneath assumption is correct?:

Saving (by reducing the wear on) the clutch and engine, and (mildly) increasing mpg by slow accelerating when hard accelerating is not necessary, can be better achieved by shifting into gears with low engine RPM's.

Low RPM will save the clutch by having less friction to overcome, and the engine by having less wear on the bearings, but there are limitations.
Quite often you can notice the limitations of the engine, by viewing increased engine (and car) vibrations, engine knocking, or the engine shutting down due to too low RPM's.

And while (an ordinary fuel) engine idling is usually between 500 and 1000rpm, the engine is not heavy taxed at these RPM's, so you won't cause a lot of wear on the engine.
Larger or excessive engine vibrations generally occur at very low (near to idle) RPMs with the throttle pressed in.

Driving or switching gears at (normal) low engine RPM, makes the switch a bit rougher, a bit harder to achieve than at higher RPMs, but with little training it is possible to achieve. Try getting into gear with low RPM, AND little crossover time (where the clutch is half pressed between the gear switch; in other words, make the switch fast, without causing too abrupt acceleration).

Switching at TOO low RPM's can cause engine to wear down faster as the wear on the bearings can be extremely high (you can see the engine will vibrate more, especially when engine knocking appears), or in worst case, can cause it to break.

The question is, HOW LOW can you go and still stay within a 'healthy' range?

I have a 2003 PT 2.4 l, and I've noticed that when shifting gears to a new gear where the tachometer reads a value below 1500RPM, makes the engine vibrate more; so I presume that would be the lowest switching point.
Of course, when accelerating faster (pressing the pedal to the metal), I'd probably raise the value to 2k RPM.

Do you agree with this reasoning?
What do you find as being the best readings for (economically) switching gears on your PT? (regardless of what engine, turbo or not).

In real life I do accelerate into higher revs (3,5k) quite often for the fun of it, so I'm not bound to drive in the above pattern; But whenever I feel like it, could driving in the above pattern be 'healthy' for the car?

Last edited by ProDigit; 02 Dec 2010 at 09:10 am.
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Old 03 Dec 2010, 04:19 am
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Default Re: Lowest RPM for switching gears (manual transmission)?

talking to an old timer i work with, a guy who has drivin manuals all his life, told me that i should switch gears around the 2.5 mark on the tach, to save gas and keep ware down, but this is his general rule, recently i lerned it shifts great at the 2 on my tach, the 2.5 is better for a bit faster acelleration, but i find switching at the 2 on my tach is comfertable and saves gas. oh, and one more thing, i usualy switch in to 5th gear around 35 MPH, it can handle that with no vibrations and keeps the car speeding up to 45 at a nice steedy pace
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Old 03 Dec 2010, 05:28 am
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Default Re: Lowest RPM for switching gears (manual transmission)?

The ideal rpm is going to vary from day to day and from shift to shift. Are you on a flat straight road or are you going slightly up and incline or even down an incline? Is the incline very gentle or highly sloped?

Back in the early '80's I had a brand new Doge Colt that had a four speed transmission with a two speed transaxle (anyone remember those?). When the car finally died it had close to 130,000 miles with the original clutch.

I think a lot more important is how much you slip the clutch when engaging or if you tend to ride the clutch.
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Old 04 Dec 2010, 03:14 am
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Default Re: Lowest RPM for switching gears (manual transmission)?

I agree with all above, I don't think there's a true rule on rpm at shift. I know that when I want to drive as economically as possible I try to keep my shifts down to where I don't here my BOV blowing off. When I drive with that kind of shift point my mileage tends to greatly improve. Most of the time though I'm shifting near red
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