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This is a modern manual transmission car.

I've been trying to change my driving habits in order to optimize my driving using this fancy auto rev match. I was skeptical at first, but I'm beginning to enjoy it. We can debate about it elsewhere.

The way that the auto rev match works, at least on my car, is right when you depress the clutch, you have a 3 second window where you can downshift, and it'll rev match for you. But if you're past the 3 second window, it doesn't work.

I admit I might have picked up coasting a long time ago which is a bad habit, but I'm trying to change my habits so that I only depress the clutch only when I'm about to change gears.

My question is...if you're in 6th gear, how slow can you go before you start lugging the engine? I would prefer to go slow enough that it won't rev too much going into 2nd. Or should I downshift into 3rd then 2nd to make the turn?

I realize it's different in different cars, but I'm not exactly looking for a precise MPH.

user1164937
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  • Can you explain to me why this isn't a driving technique question, which is off-topic for this Stack? – Pᴀᴜʟsᴛᴇʀ2 Sep 04 '21 at 23:58
  • This definitely does read that way, but I'm specifically asking here to understand from a bottom up approach. I already know the technique, but I'm trying to see to what extent based on what exactly happens under the hood (pun intended). Grey area for sure, I'll let you decide. – user1164937 Sep 05 '21 at 00:11

2 Answers2

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Consider to perform the research necessary to create the answer to your question.

When you accelerate, you have a general idea when to upshift. If you upshift too soon, your engine will lug until sufficient power is applied to get out of the lugging mode. Additionally, you'll develop a proper concept of the high end of the rpm if you upshift too late.

In reference to the comment regarding "defining lugging," one can consider that the engine is being lugged when the engine rpm drops to a point at which it is unable to provide power to the drivetrain and if unloaded with no change in throttle, would stall out for insufficient rpm. Additionally, when an engine is lugged, there is an increase of vibration and stumbling of combustion. The existing momentum of the vehicle is "returned" to the engine to keep it rotating but the rpm is incorrect for the existing conditions.

If you have a tachometer in your panel, you are ahead of the game.

For example, (made up figures), second gear will lug going into third if you shift at 1500 rpm and overspeed if you shift at 6000 rpm. Additional gear shifts may return similar figures, for the sake of this example.

On the downshift portion, you would then expect that shifting from sixth gear to second gear requires that you allow the engine to approach "lugging rpm" as you slow, but you won't be lugging the engine, as you are not applying power.

If shifting into second gear at the ground speed of the sixth gear lugging speed does not push the engine into the overspeed range for second, you should not exceed reasonable limits.

Partially related to this concept is that some will suggest downshifting to slow, but I read many years ago in either Car and Driver magazine or in an auto advice column in the newspaper that one can consider the costs of wearing out a clutch disk by so doing to the costs of wearing out brake pads by not downshifting for slowing.

fred_dot_u
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  • Can we expand upon what constitutes lugging?

    "but you won't be lugging the engine, as you are not applying power."

    So what does it mean when you've slowed to a near stop in 6th gear, and the car starts vibrating? Is it not lugging but something else?

     – user1164937 Sep 05 '21 at 00:23
  • When upshifting to 2nd at ~1.5k rpm, is it okay as long as you're staying in 2nd and not accelerating too hard? I've been taught to upshift at 3k rpm in order to prevent lugging, but is this only true when you're trying to accelerate to higher speeds in a short period?
    • – user1164937 Sep 05 '21 at 00:27
  • I created imaginary figures to provide a representation, not necessarily based in reality. If you shift to second gear at 3000 rpm, you may be in a performance model, applying maximum horsepower to the drivetrain. Typically, maximum torque is at a lower rpm, which may also correspond to maximum efficiency. If the engine does not lug, stumble, or otherwise object, shifting early is acceptable. I have shifted from first to third, because I reached the high end of rpm for first and was above the low end for third. Downshifting in this manner can work as well. – fred_dot_u Sep 05 '21 at 00:49
  • From a subjective vibration and performance observations in a 2001 Honda Civic EX, 2009 Honda Fit, and 2018 Honda Fit, I find 1500 RPM the lowest engine speed at which I'll apply any power. While unpowered (decelerating), I'll let it get down to 1000 or so before downshifting or going to neutral. – wistlo Sep 05 '21 at 15:08