They are different. Some slide more when you add throttle, some climb better, some are lighter, some have softer suspension, and so on.
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I think the top speed is pretty balanced throughout the vehicles. The rip-stick is pretty bad in general but if you time your boosts and angle well, you can pretty much fly around the map. Lawnmower tractor has really tight turning and doesn't lose as much traction while adding throttle compared to things like the ATV. ATV has nice suspension and good for controll on bumpy surfaces. I have been making alot of changes to the grip to make the vehicles more responsive without making it too easy to just turn around in the middle of a chase. Experiment with using less throttle when you need control going through trees, or using more to slide around a turn.
I believe the kids car drifts the most of all of them. The snowmobile originally was meant to have the best climbing capability, but the rest of the cars got a bit of a buff since then so I'm not sure if that is still the case.
Just connect a second device like an Xbox controller and it should go into split screen mode.