I promise this is my last question....altitudes along I-90

There's a difference between altitudes and grades. As far as actual altitudes, you'll not get much over 6200 feet as a maximum on that route. Very big difference between that, on I-90 and what you would find in Colorado on I-70, with altitudes near 11000 feet. Big difference.

However,even at lower altitudes you can have grades that are equal to what you'd find in Colorado. They just don't climb as high. And a long, steep grade can be taxing, both on the vehicle as well as the driver.