I averaged 25 MPG in my Manta (2.0L With a 40FAV).
I used to get in the 28 range with my Kadett in H.S.
When delievering Pizza in my old beater Manta (back in the early 90s') I used to get 24 around town (Heavy foot AND stop and go).
From what I have heard the GT gets about 15-20% better than the Kadeet or Manta, ESPECIALLY on the highway, due to aerodynamics.
40 MPG highway is not uncommon on a well put together high compression motor, with a good Carb (Even Solexs in proper tune can be considered a good Carb)

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As for Points VS Pertronix.. the Pertronix will NOT get you better gas mileage NOR will it get you more power. IT will save you the maintenence of dealing with points though, and it will make the motor (and thus popwer and MPG)consistant.
From what I have heard, the Low compression motors suffered in MPG. IT doesn't help that your foot has to be in them a lot more due to lack of power.
Charles