Dude . . . you think that is bad ?? I get no more than 95 miles per gas tank (city miles) and that is when I am lucky , most of the time is around 80 , heck I filled my gas tank 2 days ago , went to work 2 days and after 37 miles I am almost @ half a tank !!
What jetting do you have? which jet kit? Needles are different in jet kits as well as pilot
jets if it comes with the kit. You're going to end up with carbon problems as well as fouled plugs, which they probably now are(plugs) There is absolutely no reason not to be able to achieve 40mpg at a cruise speed of 65, 70 mph, and as much as 50mpg.
Granted, I'm no expert, but I have read every post in the RSR
forum on carbs in the past 5yrs, guess somewhere between 5k to 10k worth of posts. enough that I have seen a pattern. 95% of the time stock pilot jet is fine for just about any air kit, there have been only 2 kits I can recall that required a larger pilot jet, and quite a few who went to a larger pilot jet were compensating for an intake manifold air leak. Use starting fluid to search for leaks, it is more volatile then carb
cleaner to find a leak.
Suggest you also look at the choke, there is an article in tech section to mod it to ensure that it is closing all the wy, the choke on the stock carb just dumps raw gas into it, if it is not shutting down all the way, you are just dumping more gas.
Elevation is also an important factor usually above 1500 ft requires leaner jetting due to less air.