I drive a Mustang GT, and I love the deceleration "pop" that you get when the engine revs climb without the accelerator being pressed, so as one might surmise, I tend to downshift to a stop at lights and stop signs to hear that "pop pop rumble". Generally it's from 5th at 40 mph to 2nd, which is well under redline, but it's a large enough increase in engine RPM to give a few good pops. I haven't noticed any signs of additional wear and tear, but I figuredd I should ask those more versed in this sort of knowledge than I about it. Is there any significant increase in wear and tear on any of the drivetrain components from doing this? Things I can think of are maybe the clutch, the transmission output shaft seal, and possibly the gears themselves either in the transmission or in the rear end, but I think those last two are unlikely. I'm thinking over time I might end up clogging a cat as well, but I'm not exactly sure how quickly that would happen either.

1 Answer 1


The synchros will be forced to bear the brunt of downshifting like this

The synchromesh gears are gears inside the manual transmission designed to help the engine transition smoothly to its new RPM.

They will wear out eventually, but the wear and tear will be accelerated by downshifting without adding throttle to help the engine match its new speed.

The act of adding throttle helps by increasing engine speed, allowing it to accommodate the ratio of the lower gear without loading the synchros as much. If the throttle input is such that the engine-to-driveshaft speed ratio corresponds to the gear ratio (rev-matching), the synchro has nothing to do (zero load).

A worn synchro will result in a crunchier, more notchy gearshift. It makes shifts more difficult to perform. In such a case, refreshing transmission fluid will only have a temporary effect. The only true remedy is to replace the worn synchro, which involves pulling out the transmission from the vehicle and disassembling the gearbox.

Downshifting without throttle input will accelerate wear on the clutch as well, but not to the same extent as what the synchros will experience.

I don't think downshifts will result in clogged catalytic converters.

  • Hmm, I don't get it. The synchro is needed to bring the motor-side shaft of the gearbox to its new RPM during shifting, not the entire motor. The motor changes its RPM when releasing the clutch. Of course, sudden change to the motor RPM causes high loads on the gears and the clutch, but I don't understand what you mean by "syncros help the engine transition..."
    – sweber
    Commented Sep 17, 2015 at 8:44
  • @sweber : The key word in that sentence is "smoothly". The speed of output shaft of the gearbox (which goes to the wheels) will be governed by the speed of the vehicle. Switching to a lower gear while pressing the clutch will make the engine-side gearbox shaft spin faster than the engine. Upon engaging the clutch, there is a mismatch between the speed of the engine and the gearbox shaft that has to be equalized in some way. The clutch allows for very little slip between the engine crankshaft and gear input shaft, so it is up to the synchros to pick up the slack while the shaft speeds match up.
    – Zaid
    Commented Sep 17, 2015 at 10:58
  • @sweber : Here's another way to look at it. The gearbox in some older cars lack a synchro. Changing gears (both up or down) can be quite a jarring experience because of the abrupt change in RPM that the engine-side of the driveline is subject to
    – Zaid
    Commented Sep 17, 2015 at 11:02
  • The worry about the cats is minor, but since the decel pop comes from raw fuel making it into the exhaust and then burning, I assume a small amount of fuel makes it to the cats themselves, which I understand is not good for them, although I suppose the amount that makes it there before burning is small. Thanks for pointing out the synchros. I wonder if this is just the excuse I need to get a Magnum XL T56 :)
    – shuvool
    Commented Sep 17, 2015 at 15:50
  • @Zaid: To my knowledge, the synchros work like this: When the stick is pushed into a gear (while clutch pedal is pressed), the synchros speed up the engine-side shaft until its RPM matches the RPM of the gear wheel. Right after that, you can push the stick further into its final position, which locks the gear wheels to the shaft. Now, there is no force on the synchros any more. If you release the clutch, all torque is transferred by the gear wheels, but not by the synchros. So,the synchros wear a little when speeding up the shaft, but the high torque when speeding the motor goes to the clutch.
    – sweber
    Commented Sep 19, 2015 at 19:05

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