In late September I made a 1900 mile trip from McCall (west central Idaho mountains) to Berkeley, CA, three days of driving time total, with the first and third days almost entirely highway/freeway:
'12 SE hatch
5 speed manual
89 octane gas
day one: 13 hours through Idaho mountains and Oregon plains to Crescent City, CA - average speed appx. 60, average mpg 42
day three: 13 hours from Berkeley to McCall, via Nevada, mostly flat lands with notable mountains at Tahoe and in Idaho - average speed appx. 70, average mpg 38.