Not a problem of the scenery, of course.
You probably have set the SmallPartRejectRadius in your FSX.CFG too high, in order to ask FSX to not display small parts, like weels on vehicles. It's a setting that is usually suggested on forum, because it can increase performances with Autogen and AI airplanes but, of course, it affects everything that has small parts, like ground vehicles.
In the Addon Manager there's an easy to use slider to set it at any value. The default is 0 (nothing is rejected) but you might try to experiment is with the highest value that doesn't reject the vehicle wheels. You need to restart FSX at each try.
