The majority of interstates have a max of 6% grade but a couple others come to mind:
I-70 west of Denver:
Eisenhower Tunnel area around MP215 at 11,000' elevation has a west downhill of 7% for about 6 miles
Vail Pass on I-70 westbound also has a 7% for 7 miles
Just take your time and it'll soon be over. :)
Highly recommend that you purchase the 'Mountain Directory for Truckers and RVers - West Edition'. It come in East, too. We used it all the time because we enjoy secondary highways and 'most' mountain roads don't bother us. The first biggie is the hardest! Then you'll be a pro.
