so how far did you get on 1 tank full in 5th gear and same ? for 6th?
If you really wanna know the definite mpg answer in 5th or 6th gear....when you get to 70mph in 5th gear, set your cruise control on, reset your mpg counter, and cruise for at least 5 miles and note your mpg. Do same for 6th gear.
Your test has too many driving variables to be definitive. Unless you did your test just driving in 5th gear using a tank full and did the same in 6th. But I doubt that is what you did. You had a some street driving, stop and go, and freeway cruising, different weather conditions, etc all mixed in. You gotta be extremely anal retentive to remember exactly how you drove and where you went in the 1st tank full, to do the very same (except cruising more in 6th gear) driving patterns in the 2nd tank full to come to a conclusive result in mpg.