Hi Kiwi! Perhaps I can help explain also in addition to what Mazman mentioned. All the cars are basically the same in the physics category in order to keep the game fair and even. The karts also have the same amount of grip as the other cars but it's actually the tracks themselves which have different physics properties.
Some tracks like the rally courses have dirt physics so the car slides more. Some tracks like the karting tracks have a lot more grip as the karts really stick to the road. Karting tracks are also much smaller than other tracks so they turn much sooner giving the impression it turns more.
The only way to create physics for a track is to create a track itself in the Track Editor. Just a tip though, creating a track takes a lot of time and is something I also have spent months on a single track alone. It's not easy for sure but if it's something you'd like to learn, it is a lot of fun.
Hopefully that helps!