The odd thing about your gas consumption is that it's uniform whether city
or freeway, if I'm understanding you right.

My '84 1.9 Westy gets 15-16 around town, 19-21 on the freeway.

I find that mine is extremely sensitive to driving conditions. On the
freeway, take off 2 mpg for AC running, another 1 to 2 for driving 70+,
another 1 to 2 for headwind, and so forth. Driving 70 with AC and a headwind
I'd be lucky to get 18, esp. fully loaded.

But there should be a least 4-5 mpg difference between city and freeway.

What kind of tires are you using?