Wow, THOSE are some impressive numbers -- all of them!
• 628 miles on a single tank!
• 13 gallons on a 12.6 gal. tank (I know, I know...it really can hold 15.6 gallons)!
• 48 MPG = 628 miles / 13 gals.
• 62 mph 'cruise control' speed.
• TWO passengers plus luggage.
• A/C on all the time.
I'm betting it's due mostly to (a) constant use of 'cruise control' to (b) maintain that constant 62 mph speed.
What air pressure are you keeping in the tires, 35 psi?