Best I managed on a Mk2 Midas was 58.5 mpg @65 to 70mph Over a 200mile run, Satnav figures) from a 1380 1.5 rockered Twin SU economy engine. It also used to return a regular 45 to 47 on the trip to work. This engine used a custom curved 65Dm4 dizzy and custom needles.
The bottom line is you need some sort of mapped ignition to get the best fuel economy and you need proper custom needle, none of this filed rubbish, a proper machined item that produced the fueling where you need it. It is perfectly possible to get fueling at around 1 to 1.5 % CO for cruise then richening to 5.5 to 6% when you floor the pedal. How do I know? This is the aim when I setup a car, on a rolling road. In 95% of cases, it won't be done with an off the shelf needle though. Having a vacuum gauge and using it while cruising is a must.
If any one is interested, an economy engine build can be supplied by MLmotorsport 01474 825123 for more details, and tell them I told you to call. The bottom line is though fuel economy is only as good as the driver.
AC