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AC collisions suck. There is not much you can do about them, but you can mitigate the problem by at least having your colliders set up properly.
The KN5 collider is used by ground collisions, wall collisions and car contact collisions.

Prerequisites:

• Blender3D
• Basic knowledge of Blender controls.
• Kseditor
• CM with developer mode enabled.
• A car

The car we will be working with is my friends 300C. The collider is from the Formula K, so the car behaves funnily when you drive it around. Step 0. Extract the KN5 to get a reference for the shape of the car, and import it to blender. Step 1. Create a box in the origin of the scene and scale it around to fit roughly the bounds of the car. Step 2. Block out a basic shape of the car. Do not make it too detailed and make sure the model is manifold. (waterproof, no holes) Step 3. The alignment.
Because different 3D programs use different notations for the axises, at some point in the pipeline from FBX to AC, the model gets rotated 90 degrees. This is why colliders come out wonky/incorrectly rotated at times. To counter this, do:
First rotate the collider model -90 degrees on the X axis. CTRL + A to apply rotation transform. Also apply scale transform, if you have those.
Then rotate the model +90 degrees on the X axis. Your collider model should now have origin 0,0,0 rotation 90,0,0 and scale 1,1,1. Your collider should now export with the correct alignment. Step 4. Add material. The material name can be whatever you want. Step 5. Exporting and kseditor.
Use these export settings and everything should go fine. MAKE SURE YOUR COLLIDER OBJECT IS THE ONLY SELECTED OBJECT IN YOUR SCENE (ORANGE WIREFRAME). In Kseditor make the collider object active and select material GL. Export KN5 (car) and name it collider.kn5

Step 6. Complete(?)
If you have worked with a Kunos pipeline compliant car that has been aligned correctly, this is where the guide ends. Your collider lines up with the car in CM, everything is fine. (Do note, that basey and visual offset rotation in car.ini may affect the rake of your collision model.)

As you can see the collider shape has changed from the last time we saw it. But it appears something is not adding up! My friend Tes has been slipping and has not aligned the car to be above 0 cordinate on the Y axis!!!! So the collider turns out slammed to the ground, as the collider function in AC always assumes the model is correctly aligned above the 0 cordinate.
This is a simple fix:
Re-open Blender (you DID save your blend, RIGHT?)
Select all (press A twice) and drag everything up until the tire lines up with the green axis. After this, Apply the location transform (CTRL + A). DO NOT APPLY ROTATION OR SCALE AGAIN.
After this, repeat instructions in step 5. Now you should have a working 3d collider! And it only took you 1 hour and 32 minutes. Colliders.ini

Colliders.ini are simple box colliders that only collide with road surfaces. These are useful for things like collisions for front splitters, diffusers, or squat-plates.
For a simple road car application, just make the collider span the general bounds of the floor of the car. Collider thickness does not need to be more than 0.1.

Collider alignment troubleshoot
Problem: Collider comes out rotated 90 degrees the wrong way, despite looking fine in kseditor.

If you meshed out your own collider, this should not be a problem, however if you used the convex hull modifier method from Tes' tutorial, you may encounter the following: Delta transforms.
In simple terms: When an object has a child, and it is rotated, the children do not technically get transformed. However they still move along with the parent, so they get a delta transform. Chances are, that if you have imported a kn5->fbx file, your objects may have deltas. This is thankfully a simple fix.
In Blender 2.8, just press CTRL + A -> APPLY ALL TRANSFORMS. Then re-apply rotation transform by following step 3 in the guide, if you need to.

In Blender legacy (2.79b), the tools for handling deltas are very primitive.
In 3D view, make sure the T menu is open (lefthand side panel), then make sure your object is active and clear transforms with ALT-G (position), ALT-R (rotation), ALT-S (scale). In the lefthand side panel, make sure to click the CLEAR DELTA tickbox for each time you press the binds.