We do alot of 53' race haulers. The unique thing about race haulers, are that when they are at the track, they setup and awning and it now doubles as a hospitality area. So you have sponsors and 'important' people funding the operation, sitting within 10' of the prints/wraps.
We've gotten away with 40 dpi often enough... but 100 is really preferred.
Every photographer says the same thing... 'yeah yeah, we can get you whatever res. you need' ignoring anything we tell them beforehand (1/3rd actual size, 300 dpi).
Then afterwards they're dumfounded... wondering how billboard companies can use their photos without any problems lol.
Um... people don't sit 10' from the billboards.