Well, 5 inches is closer to 6 inches, which is 1/12, but they're really too small for 6" figures.
Cason for Inanimate Objects did some amazing DC Animated customs with these vehicles though:
http://pilliod.net/customs/batmobile-retro/http://pilliod.net/customs/harley-quinns-car/Which inspired me to try to use one for modern Joes. It's not going to work straight out of the box, but I was REALLY happy with how my "ARES Dictator" came out after swapping in an interior from a Maisto model.
http://www.joecustoms.com/customs/customs_specific_item.php?kind=vehicle&thumbs=off&uid=2027&id=828They are great vehicles, but the Taxi Cab is a BEAST. The "limo?" is small enough it works as a large 1/18-ish scale vehicle. The Taxi could make an amazing art-deco tank or something, but it's going to dwarf even a HISS II in length and width. Expect to have to take them apart (easily unscrewed) and severely modify the interiors if you want Joes to fit properly. I cut out the original interior on my project and completely replaced the seats and added a dash from a Stinger.