When I travel from Sacramento to the Bay Area on I-80 at 65mph, going a little up and down, surrounded by cars, trees, hills, etc., I get over 50mpg.

When I travel from Sacramento to LA on I-5, going 70mph, out in the barren open flat land, surrounded by nothing, I get around 45mpg.

I think your numbers are pretty reasonable. As has been mentioned, if you were able to go 65mph on that stretch of 101 (that's usually the speed limit), you'd probably get closer to 50mpg. That's why I personally prefer to take 101 rather than I-5 (not to mention the more pleasant scenery on 101).