I was getting crazy elevation data on some segments of the Sierra Cascades (up to 60% gradient!) but didn't mind them, assuming that elevation of a road carved in a canyon could be off because the satellite reading was a few meters left or right. But then I was really puzzled by a very steep segment on the supposedly easy Eurovelo 6. Turns out that the elevation data refers to the terrain (obvious, isn't it) while the trail goes through a tunnel...

Anyone knows about a source that would provide road elevation? (or flag tunnels on a course)

ridewithgps.com seems to be more accurate than some others wrt elevation data

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.