just tried ridewithgps on the segment Wawona -> Lee vining. at km 41 it shows gradients of close to 30% at the Yosemite tunnel...

i later found this page on the rideWithGPS site, where we learn that premium subscribers can manually flatten sections of a course. The issue of tunnels/bridges is frequently raised (lots of hits on a google search) but no practical answer provided.

could be interesting if riders with bike computers fitted with barometric altimeters were to submit their tracks.