There is NO WAY a 21" passenger car will ever be able to go around anything less then an O-54 or even an O-48 minimum and look good. In reality the 21" cars start looking okay on O-72 curves. But they start looking better on O-80 and larger curves. 16" passenger cars do not look good with anything that is bigger then a scale F3 or F7. The big six axle passenger diesels, along with F3's and F7's all look good with 18" and 21" passenger cars.
With respect, that's your opinion. There are many folks who don't care about the look around curves. They just want matching passenger cars to match the engine. When I'm visiting my friend with O54 curves it doesn't bother me a bit....but that's just me. It's pretty obvious the OP doesn't care either. To each their own.