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The scrubber street sweeper can be converted to a zombie killing beast at rim jobs.

Are there any other vehicles that can be converted for zombies?

spiked bumpers etc.?

The Genkimobile doesn't require any modifications at all to be perfectly suited for zombie annihilation though it is a good go-to choice if one has the Genki Bowl VII add-on.  One can never really go wrong with a vehicle with bulk, but it does feel like a cheat to say a Bear, since they are already armored vehicles with turret mounts on the roof.

Too bad that Firetrucks don't have flamethrowers otherwise they'd be ideal for the task, even when used against zombie brutes.