If you have less skill, choosing the most efficient kart won't save you. It can help to squeeze some time, but the difference is often tiny, especially in full-game runs.

You can see kart's length by launching the game with the --kartsize-debug command. Longer length = shorter turn radius. Also, smaller width means smaller hitbox (generally good, you need to better aim for nitro/bonus boxes, but you'll more easily avoid bananas/bowling balls/gums).

[big]0.9.0 and older[/big]

These versions don't have kart characteristics : all the karts have the same speed, acceleration, etc.

The main difference in karts is the turn radius. A short turn radius allows to batter take turns, but a long turn radius allows to easier abuse skidding in more straight portions ; so the optimal value depend on tracks and strategies.

[big]0.9.1 to 0.9.3[/big]

The kart characteristics introduce some significant differences between kart classes. They don't impact turn radius, and the differences here remain linked to kart characteristics.

Heavies are basically useless, completely outclassed by mediums. Don't use them.

Medium are the reference : they perform best on most tracks, having the better acceleration/max-speed/nitro combination.

Lights are too slow to keep up with mediums on most tracks and on full-game runs (if you have enough skill to not crash a lot, of course), but their higher acceleration means they gain some time at start and can do a full-180 easier (the turn radius is independent of kart class, but high acceleration allows to switch from going backward to forward very fast). This makes them useful on some level runs exploiting game bugs by changing direction and driving backward up to a checkline. They also can be on par with mediums in very short races (1 minute or less), because their ~1s starting advantage becomes then significant (for example, it's possible to beat my current Tux time in Zen Garden with a light kart).

[big]Kart's turning characteristics (0.9.3)[/big]

Among light karts, Wilber has the shortest turning radius. Kiki is not far, but has a slightly bigger one, then Hexley. The light Sara kart has a significantly bigger one. Considering you should only use lights in track where you need to do a full-180, Wilber is probably the best unless it really matters that he can't skid as straight.

Among medium karts :

• The longer turn radius goes to Adiumy. So big that you'll probably have some trouble with it, though it's possible to compensate with skidding on numerous tracks. The AI can't handle it and get catastrophic results, but it may be the best kart for some ILs.
• The shorter turn radius goes to Konqi (which also has a high width).
• The order (bigger turn radius to smaller) for the others is Emule (small width) > Sara the Wizard (very small width) > Gavroche > Gnu > Suzanne (high width) > Nolok > Tux > Xue (very high width)