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The most direct route is as simple as simple gets. You get on I-90 and drive east. For Detroit, you can detour at Chicago onto I-94, and if your passport/visa and rental car allow it, cross through Canada to get to Niagara Falls, otherwise head back down to I-90 and drive around the south of Lake Erie.

Now, you could do that in as little as about 5 days, so with 8 weeks, you certainly do a whole lot more than just take the direct route. That really depends upon what you want to see and do with your time, especially in regards to the Rocky Mountains, which stretch from Canada all the way to New Mexico. The more you tell us about what you are thinking, the more others can provide for additional suggestions.