Marco:
Your MPGs are a little low. But are your gallons Imperial @ 160 oz or US?
Road and Track would be calculating with US @128 oz so that if you
are convering to Imp gal the Road and Track milage is actually 160/128=
5/4 X 17.5 to 21= 21.8 to 26.25 miles/Imp gal.
Make sure your vacuum advance on your distributor is holding vacuum when
you cover its vent hole and your O2 sensor is working w/a % meter on the
test wire between the W/S washer resevoir and the left inner fender.
The only 505 I have driven extensively on the hwy.is and 85S SW w/auto it
never got more than 22 MPG Imp. but the SW has a very low final drive .
Bob Bruce