Hello all, we're just about to play a game where my mate has built a vehicles deck, but never played the crew mechanic before. He's just asked what happens if he uses 4 power to crew 3? Does he get to use excess power to crew something else? Kind of like trample?
The cost to activate "crew N" is "Tap any number of other untapped creatures you control with total power N or greater" (C.R. 702.121a). Thus, for example, to pay for Ballista Charger's "crew 3" ability you can tap—
three creatures you control, other than Ballista Charger, with total power 6 (e.g., three Runeclaw Bears), or
four creatures you control, other than Ballista Charger, with total power 5 (e.g. three Zealous Guardians and one Runeclaw Bear),
among other possibilities. However, in general, tapping creatures with greater total power than N this way has no bigger effect than tapping creatures with total power N this way.
EDIT (Feb. 11. 2022): Edited to conform to rule changes with Kamigawa: Neon Dynasty.
Basically nothing. That “extra power” is just lost.
The creature doing the crewing is NOT attacking so Trample is meaningless.
A creature can only crew one vehicle
(multiple creatures can crew a single vehicle).
This is not entirely accurate. If you manage to untap the creature you used to pay a crew cost, you can then use it to pay the crew cost on another vehicle. For example, if you tap hateflayer to crew a vehicle, and then use it's untap ability you could then use it to pay another crew cost.
Cheers
EDIT (Feb. 11. 2022): Edited to conform to rule changes with Kamigawa: Neon Dynasty.
The creature doing the crewing is NOT attacking so Trample is meaningless.
A creature can only crew one vehicle
(multiple creatures can crew a single vehicle).
This is not entirely accurate. If you manage to untap the creature you used to pay a crew cost, you can then use it to pay the crew cost on another vehicle. For example, if you tap hateflayer to crew a vehicle, and then use it's untap ability you could then use it to pay another crew cost.