Seems, conversions have been to create easier than new special software for the Atari.
Even Lucasfilm Games did it that way , by just scaling the available software down to fit to the hardware specs. of the small 8 bit.
A Test Drive with playable framerates would be fun enough.
- Yeah, Test Drive is only filling about 40% of the screen with the 3D world (the rest is cockpit+HUD), so the fillrate cost would be minimal.
- Plus, unlike StuntCarRacer, TestDrive's 3D space is very limited (far from the generic 3D meshes of SRC)
- it also appears to be constantly repeating same few identical segments (straight, 2-3 bumps, 2-3 versions of curves), so a lot can be actually precomputed and just pre-stored at loading-time without imposing a run-time computation cost
- at the very least, 25-33% higher framerate than StuntCarRacer is easily achievable during most of the driving - e.g. without opponents (which will, understandably, for those few frames, bring the framerate down accordingly)