A normal MTB/cruiser rim is 26" diameter, and the usual cruiser tires you find are about 2.25" wide.
Some of the cruiser bikes (Felt, Electras, ect) come with 3" wide tires--but these are for 24" rims.
I first tried my engine kit on a "high performance" hybrid bike that had 1.5" wide tires,and the ride at 27 mph was really too harsh (for that bike frame). So I need another bike, and I like these bikes with the 3" tires (I like the Felt Chief). But there are a couple concerns with the fat tires....
....One concern is flat-prevention. I doubt any of these cruiser tires are kevlar-belted. Does Mr Tuffy make strips wide enough for 3" tires? I'd guess not (though I haven't looked really). I've seen one bike that had the 24" dia 3" wide tires up close, and the tires looked pretty thick, at least 3/8-inch of tread on there. They were heavy too, but for a motor-bicycle that in itself is not so important an issue I guess.
....Another matter with the 3" tires (and bikes that run them) is that the bikes often don't have full fenders. Part of the reason people like the cruiser bikes is because they have fairly-fat tires, but they also have good [functional] full fenders included too. If you live in a desert maybe you won't have this problem--but if you ever have to ride when it's wet, you're either going to have fenders, or you're going to wish you had them.