The "legal" restriction is actually a technology limitation. The highest an IBOC "HD" channel can be injected into the analog system is -10dBc. The original limitation was -20dBc.
In flat land areas, expect an HD signal to go about 2/3 as far as the analog counterpart. In mountain/hilly areas, closer to 1/2. Digital signals don't bounce nearly as well as analog. With analog, you'd get some hissing or popping (multipath, signal comes to the receiver from 2 directions), with digital, if it bounces, it scatters.
In a vehicle improving reception is a big challenge. First, you have no height, so getting the antenna up in the air isn't really an option. Second, you have electrical "noise" to contend with that's going to knock the signal down even more. 2 things that start out against you. A good tuner can help with these. In my truck (durango) i get great HD reception for about 50-60 miles around the transmitting antennas in town. But i'm in flat lands too. My same tuner in my girlfriends Fusion gets maybe 30 miles of good HD reception. Her car has a lot more electrical noise than my truck (not sure how, but it does).