Changing Gear Without Stepping on Clutch

[sphinxz]

Just rev match to shift into your desired gear. First saw this on Top Gear many years back, been using it as a party trick whenever someone rides my car ever since.

Bad for the synchros of your transmission, but pretty cool trick every now and then, and very useful skill to learn if you ever have to deal with a broken clutch.

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So how would you know its time to shift? Does it vary in every gear and car type? Or is there a standard rpm?