The .acc or .ac files are the 3D models actually loaded by Speed-Dreams.
I have only worked with them as part of track generation/creation, so this may not apply to cars.
The .acc files are 'compiled' or 'optimized' versions of the .ac files.
The .ac files are generated by sd2-trackgen. They can be edited with the AC3D software or they can be imported/exported into Blender (specific version?).
I think that the .acc files are generated from the .ac files using sd2-acc. I have never done this, I have always just used the .ac files as none of my tracks are ready for distribution.
There is a brief description here:https://sourceforge.net/p/speed-dreams/wiki/ArtworkFileFormats/
Hopefully some of the 3D moddelers out there will correct anything I got wrong.