How does ignition timing change affect MPG

[58DeSoto]

advancing the timing increases torque and moves the torque peak to a lower RPM this will increase mph. advance the timing to the point of pinging then back it down a bit. this can be done without a timing light. if your car has mechanical advance (under the dist cap you will see metal weights held with springs) you can put weaker springs in place of the stock ones this will allow your timing to advance when you drive faster when the load is lighter