What route should I take from Dayton, OH to Ashford, WA?

Google Maps gives three options: I-80 W (2414 miles), I-94 W & I-90 W (2407 miles), or I-90 W (2379 miles). What I am concerned about is that my car has only a 4-cylinder engine with very little power going up mountains. Any info I can get about these routes would be very helpful.