It depends a lot on where you live. At sea level most wings will post low 40's to high 30's at highway speed but in high altitudes the efficiency goes up due to leaner mixtures and 50 is not uncommon. In general 38 is a good number for average.
I drive 56 miles a day round trip to work and back and about 50 of those are open road. On 2 lane highways I run 62-65mph and on the interstate run 70 and average right around 40 mpg, some weeks are better and some are worse depending how my grips the throttle LOL