Ignition timing for best gas mileage

I have an 87 Dodge Dakota V6, originally equipped with a Holley 6280 electronic feedback carburetor. The mixture control solenoid in the carb went south a while back and is no longer available so I replaced the
6280 with a 2280, which is the non-feedback version without the mixture control solenoid.
Recently, after installing the new carb and an MSD spark booster, and adjusting everything to factory specs, I made a 200 mile trip which was about 90% freeway and with a 1/2 mile gain in elevation.
I got 22.7 mpg. And this is with a heavy duty lumber rack and a 2'x2'x4' toolbox full of tools and starting out with a full tank of gas. The truck had about 113,000 miles on it.
I was really dumbstruck at the gas mileage I got on this trip and then started to wonder how I could better it. The factory manual says disconnect all the ignition timing inputs to the computer to set the spark to 7 degrees BTC. That's where it is now.
Which way should I tweak the timing to get better mileage? Should I go 5 degrees BTC or 9 degrees BTC? I am especially interested in getting better city mileage with lot's of stop and go driving. Can I expect this by tweaking the ignition timing?
--

McCain -- it rhymes with Hussein, Bahrain, cocaine, insane, wolfbane,
chest pain and chow mein.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Samson wrote:

--

There are some power - and therefore economy -
by maximizing the spark timing. That doesn't
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.