I have an 85 Carrera with 131000 miles. It gets 18-20mpg combined (mostly highway). I think it needs a new O2 sensor which is one of the most expensive for that model/year ($200). I have purchased a generic 3 wire O2 sensor ($45) and will put it in soon to see if it helps the mileage.

 

Stock 84 (ROW) with a Steve Wong chip, just filled up and got 17 mpg in mostly LA Freeway driving.