I bought a hubcap set for my '25 Touring, from Lang's, but it does not fit, it's not large enough... what is wrong with my car?
How are the threads on your hub? Sometimes just a couple buggered threads will prevent a new one form starting - has the outer cup ever spun in the hub? If so, the threaded area has been expanded.
I had the same problem on one of my wheels. There's a tool you can buy to chase the threads, but I didn't want to spend the $100, so I used a thread file. In this discussion Wayne Sheldon shows you how to make your own hub threading tool:
Measure the hubs. Could be that they're TT hubs.
TT's hubs are smaller?
No, here are pictures from Fred Dimock showing TT and T hubcaps, the TT rear wheel hub cap is to the left in the pictures:
I think a thread file would help. It can be very useful in many more areas when repairing a Model T. http://www.ebay.com/itm/KASTAR-2665-SAE-THREAD-RESTORER-FILE-11-24-TPI-8-SIZES-/310866271327