[Katlaw]

I remember back in 86 I was delivering pizza with my 85 Shelby Charger (AH!, Memories), anyway. I got much better fuel economy running premium over the lesser grades. In fact it made up the difference in price to run the premium.
I also know that my 87 GLHS with a heap more power is doing even better than the 85 ever did for fuel economy.

Law

 

[Katlaw]

Again, back with my 85 and delivering pizzas, I did mix 1/2 and 1/2 between 87 and 92 Octane with the equivalent power and slightly better mileage than either grade alone. That might just have been in my teen-age brain tho. I only did it for 3 tanks because I thought it was a pain to do and I was rolling in pizza tips...LOL

Best places to start looking for improved fuel efficiency is
1) Clean air filter
2) Cleaned and properly gapped plugs (hotter too maybe?)
3) check that timing is bang-on

I may just be talking through my hat though. I'm sure there are lots of other guys with more/better info that would be willing to help you.
mpgmike is a good source of information and he has lots of experience working on the Xprize stuff.

Good luck
Law