If its a straight c you will be better off at 65 if it a B+ it is a little more aerodynamic and will handle the wind better IMO. We have a B+ and it cruises just fine at 70 and we have had it up to 75 for some straight periods. Mileage is subjective, I run a 5 star tuner and average about 9 and on straight and level can get 10 (neb, colo, kansas). We tried the 60 and 65 and found mileage did not improve enough to drive that slow when on interstates. Yours will vary but like I said we have a B+ and do not have the big overcab sail to deal with!