Proper 400/440 highway mpgs?

I have found my bike gets almost exponentially better fuel economy in the city compared to the highway. I know these bikes are not exactly designed for highway use, but the difference is so incredible that it's just like "Wuuuhhh?!?!" LOL

**I find I get mid-high 60s to low 70s in the city (riding slightly out of the power band turning 3.5K at 40mph), whereas I only seem to get about mid-high 40s when I travel at 60mph (5.2k), and that drops to almost the 30s if I dare go around 70 turning about 6.5k (which I don't even do in my car...in the 75 zone).

I just wanted to know if right around 50 or slightly below sounds about right for a highway figure?

**These are the figures when my bike is running right, I am getting less due to an intake gasket/carb manifold leak and a spark plug that the crush washer seems to have gone out on (will leak gas when running for long periods of time....it is tight, I have checked that much.)