I don't remember who here said it first (not me, can't claim credit), but a good rule of thumb when bump-starting is use the gear that's two higher than what you'd normally use for that speed. E.g., in my bucket, if I need to bump-start at 25mph, that's normally 3rd gear, so bump the car in 5th, depress the clutch, and select 3rd to continue driving.
Another consideration: The faster you're going, the less aggressive you can be with engaging the clutch to spin the engine. If I'm trying to bump the engine at 20 mph, I'll put it in 4th and let the clutch out pretty briskly and pretty far, to make sure it engages firmly to spin the motor. But at 50 mph in 5th, I can just "feather" the clutch out a few inches into its travel, very quickly, and the engine catches.
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