How much city driving do you do? I know that in my truck that if i do ALL highway, i get almost 20mpg, but during my normal week routine, I get probably 16-17mpg because its 50/50 city/highway.
Also are you auto or a 5speed? With my dads r50, I get the best mileage when I drive it because I shift at 2500-3000. If you're an auto, leave the overdrive switch on all the time and don't gun it at the lights, that uses a lot of gas. Also try to use 55mph roads instead of 65 as, or at least in our r50, the gearing is such that at 65, the engine is revving at 2800