They're collectors only interested in collecting & trading not actually driving the cars. My fav is when one of them asks "Has the car been sat in?" Really? I have a newsflash for them that none of these cars will ever get sat in because it's a video game.
Yea after so many miles, engines start to loose hp and after so many engine builds might be 3 or so they will drop a few bhp, really the ucd is lame as heck with the cars with a billion miles on them. And engine rebuild should rebuild the engine good as new, especialy some cars are so damn annoying to find in the first place, and other cars are one offs, that you cannot replace at all.
One of my online buddies has a 787B stealth with 30000 miles on it. After a few rebuilds he says it's permanently lost a few bhp. I didn't ask how many I'm afraid.
Cars do occasionally appear in the UCD with 0 miles on them. I've seen the Veyron with 0 miles on it for sale before.
The effectiveness of engine rebuilds declines with age. I learned that from my FGT, a primary grinding car for Bob. I'd really love to hear more about the mechanics of it though-- exactly what mileage results in decreased engine rebuild effectiveness.
i did an engine rebuilt at around 10,000 miles on my Formula GranTurismo and its max power is now 932hp instead of 935. it was the 2nd rebuild i had done on the engine (the first being immediately after buying it), and i had changed the oil after every race (a standard practice with all my cars).