I have a 1980 Suzuki GS550L and it's geared for 55mph and supposed to get about 55mpg but the speed limit now being 70 it no longer gets 55mpg it now gets about 42mpg any suggestions on how to fix this dilemma?

I had this Vstar 250 which after the 800 miles were put on the bike it was routinely delivering 83 mpg. Now I have this 1300 Vstar Tourer and it has 800 miles on it and it's routinely delivering 51 mpg. Location of tests, Oklahoma and Kansas. Charlie NEXT CUSTOMER PLEASE!!! SOUND OFF!!!