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I have a ford fiesta with a manual transmission, and often when I'm driving gently, I'll take more than a second or so to change gear after I depress the clutch pedal. I press the clutch pedal, slowly pull the car out of gear and as I go to put it into the next gear, I'll kind of hold it there, and it will get pulled in by itself (not fully by itself but I'll gently give it some force). I find this quite had to describe, but its almost like the next gear pulls the gearstick in. This generally results in a very smooth gear change with minimal resistance. The only time I get any resistance is when I'm at quite high rpm, say 5-6k and I try to shift to the next gear I'll be fully blocked out until the rpms die.

Othertimes, when I feel like booting my car around or feel like accelerating quickly, I'll perform my gear changes much faster, so clutch in, next gear, clutch out in under a second. When I do this, I feel some resistance before going to the next gear, but since my goal is to shift quickly, the force I'm applying pushes straight through into next gear.

My question is, does the second method of shifting put more wear on my synchro's than the first method?

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Yes. When you change gear slowly, the input shaft rpm has time to drop off before the synchromesh engages, so the speed difference that has to be overcome by the synchromesh is small.

When changing gear quickly, you're using the synchromesh to brake the gearbox input shaft.

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    Assuming you have the clutch disengaged, how would you be braking the engine? Only the gearbox input shaft and part of the clutch should have to change speed.
    – Ives
    Jan 30, 2017 at 13:40
  • you're right, of course.
    – Hobbes
    Jan 30, 2017 at 13:59
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    The answer could use some extra details here. If the RPM's are a very close match, the wear on the synchro is minimal, regardless of how fast the shift is done, because there's just not much mismatch to take up. So it's true, but in a well matched shift, the difference is inconsequential.
    – Leliel
    Jan 30, 2017 at 21:38
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Under "normal" driving conditions, the synchromesh should need only a fraction of a second to allow an upshift to engage. Under hard acceleration, you will rev the engine higher before initiating your upshift, so there is a higher speed difference for the synchro to take up, so it can take a bit longer. Downshifts can take longer; for upshifts, the input shaft and clutch plate has so slow down to the proper speed, but for downshifts the input shaft has to be sped up. The more work the synchro has to do to match speeds, the more wear on its friction surfaces.

If the synchro seems to take longer than it ought to, it could be because the clutch is dragging (not fully disengaging). If the clutch mechanism uses a cable to connect the pedal to the fork, it could be out of adjustment; if it is hydraulic (analogous to the brake system), it may be low on fluid or need to be bled.

FWIW I've sometimes found myself not depressing the pedal far enough... enough that there is no useful torque transmitted and the gear shifter can be moved into neutral, but not enough to properly free it to allow the synchro to operate for the next gear; it doesn't take much clutch drag to overwhelm the synchro.

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