[olddog]

So it is has never been perfectly tuned. Now you may have had the engine, as it exists, tuned to its best possible state.

Your engine, when cruising down the road, at a very light load between 1500-2000 rpms, would make its maximum brake torque (MBT) somewhere between 40-50 degrees advance. If you were to tune it to the correct advance, at these conditions, the car would accelerate or you would have to back off of the throttle to maintain the same speed. This would result in better fuel mileage.

If you want to prove this, fill your tank, start your engine. Set the initial timing to 20 Deg. Now do not step hard on the throttle, as you have way too much timing for a medium load, let alone a hard load. Now take it for a light cruise for about 50 miles. Return the timing to 10 Deg before you shut it off, then fill the tank, and calculate your MPG.

PS
The idle will likely go up quit a bit. Just for this quick test I wouldn't bother turning it down, but it might be safer to do so. Count the turns to make it easier to put back, if you do. This fact also proves that increasing the timing is increasing MBT at this extremely lite load.