I have not driven where you are talking about but my drive over the Colorado passes includes from Silverthorn to the Eisenhower Tunnel a 12.5 mile climb at 6% grade. Never had any problem running at 65(speed limit) over this pass, no slow down, no high revs. Car ran great. I have made this trip at least 6 times (round way trip). Also, I go over Vail pass 7% grade but not a continuous climb all the way to the top as Eisenhower is.