Well that depends..... I did trail ridge road in Colorado from Estes Park - about 5000' of climbing and it gets over 12,000'. It was a hard ride but not really hard. I have done Lizard Head Pass at Steamboat Springs and it is 7 miles of 7% - it has a long "look out for falling rocks" zone and as I rode along I could hear small rocks rolling down the slopes - you don't get that in cars. I did Teton Pass in Jackson Wyoming - it is 10% for around 5 miles - and that is why they invented small chain rings. I think the hardest climb I did was Bobcat Pass in New Mexico - the last 3 miles were probably 7% but I wasn't in really good shape which is why it was hard. Long climbs + high elevation + inadequate training = that tired feeling.

I climbed Red Mountain Pass in Colorado from Silverton to Ouray. When I hit the steeper part of the climb it started to rain. I put on my rain gear, tights, long sleeved jersey etc. The rain came down in sheets with lightning. I could look down at the steep drop on my right. As I got higher and higher (peaks out at 10,600'), it got colder and colder. At the pass, snow was sticking just a few hundred feet higher up. I knew that from the pass to Ouray it was about 5,000' down and no pedalling. I figured that I did not want to do the descent in the freezing rain. My room mate on the tour did the descent (I SAGGED). I met him at the bottom. His face was blue. He could not talk. I took him into our room. I undressed him and stuck him into a hot shower - having felt the water first with my arm to be sure that I did not cook him. After that he could talk and dress himself. We went across the street and he gulped down 2 hot chocolates. We then went to the municipal hot spring and he spent an hour in the hottest pool 105 degrees.