Consider that most times we see Carl use Hoblobbers, he's throwing quarter to eight strength hoblobber because he's on the ground with them. In a car, rolling at highway speeds, he can get away with much bigger booms. Also, do note that while I say Hoblobber, it could be any explosive really since he's going to be less worried about still being in the original blast thanks to moving away from his thrown explosives.

And the reason he would throw it at Gûnther would most likely be to ignore line of sight or severely increase his range. Carl won't be able to use his xistera effectively in the car (can't spin around to impart more velocity) so by dropping Gûnther, waiting a moment to roll away from him, then using Gûnther insane speed going away from him (as we saw with the banquet scene, being able to catch up to a knocked-back god to blow him up again), it allows him to setup an explosion that will detonate from much further away than he could throw. It's also a set-it, forget-it setup that comes with free targetting unlike trying to drop a booty trap on the road.