Wow wow wow! Only 2% maximum?!? That's fantastic!!!
I'd jump right on the MPG Remedy ASAP, the concoction that lowers the surface tension of gasoline so that it atomizes more completely at the injectors, allowing for a better air/fuel mix in the combustion chambers. I'd wager your fuel economy goes up by 1-3 MPG just from that alone, if not more.
As far as no oxygen sensors on the exhaust system...how many spark plugs per cylinder does this engine have? If there are 2 as I suspect, one of them fires on the power stroke, and the other fires on the exhaust stroke to burn whatever fuel might be left to clean up exhaust emissions. I'll bet you could find the firing pattern/sequence and then move the wires around on the spark plugs so that they both fire on the power stroke and burn all of the delivered fuel when the engine needs it most and further gain power and efficiency (from not hitting the accelerator as hard to get moving). Opening up the spark plugs for bigger, stronger sparks will only help this.
What's the compression ratio of the engine?