6 banger mileage?

I have a nearly stock 73 with a 225 /6. I get ~ 16 mpg around town and ~ 25 mpg on the road. The engine has nearly 300 k miles on it and it has never been opened. The carb was replaced with a reman that I later rebuilt. It is jetted with a 51 main jet. The deviation from stock is replacement of the stock exhaust manifold with one that has no heat provision for the carb and intake manifold. Rear gear is 2.76:1