Wheel Controller 3D is a complete alternative to Unity's inbuilt WheelCollider. It allows for more realistic vehicle behaviour, complete customization, and 3D ground detection.

If looking for a complete vehicle physics package with Wheel Controller 3D included, check out the NWH Vehicle Physics 2.

v10.16f - Major rewrite of Wheel Controller 3D with improvements across the board.

Features:

3D ground detection.
Clean API standardized through WheelUAPI.
Easy to set up.
Highly optimized and suitable for mobile devices.
Around 0.006ms of CPU time, per wheel, per physics update, on a desktop CPU.
Full C# source code included.
Fully customizable via the custom inspector.
Runtime adjustable spring, damper, friction, geometry, etc.
Adjustable rotation, camber, and caster.
Pacejka friction model with support for different surfaces.
Friction preset system for adjusting friction characteristics.
Tire wear simulation.
Suitable for different kinds of vehicles: cars, trucks, motorcycles, tanks, etc.

Try it out:

Demos

More info:

Wiki
YouTube Channel
Forums