If we go with a 6 gallon tank and the stated 340 mile total range we get 56.6 for the engine and battery and 50 for the engine alone.
How they are getting big numbers is using shorter trips where the first 40 mile use no fuel. So they could say you went 80 miles and only used 3/4 of a gallon of fuel so you got 106 miles to the gallon.