Well, there is Metropolis Street Racer, which lets you buy and colour different cars, and race them around cities - very good, like a mix of Gran Tourismo and Need For Speed...
But for actual customization (setting gear diffs, shock quality, etc.) Sega Rally 2 seems to have the most options - but the game experience is far too "Arcade" - you have limited time to get to the next checkpoint - crap like that.
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