Need some advice
 

So I have an 88 4runner 22re and I recently rebuilt the motor has about 750 miles on it and when I first timed it I had the timing set at 14 BTDC because my tps was not adjusted and when I shorted the diagnostic ports it would not idle down. So I readjusted my tps and got it set correctly and was able to base time the motor correctly, I first set it at 8 BTDC and on my last to tanks of gas have got 16mpg. Before when it was not base timed correctly and set at 14 BTDC I was getting 20 mpg consistently on about 4 tanks. After checking my mileage today I set it to about 6 BTDC and am waiting to see if my mileage goes up. This is all city driving by the way. My question is did I go the right way by retarding it back closer to the stock 5 BTDC? If my motor was completely stock I would just set it at 5 and not worry about it, but its not and I know its capable of better mileage so, any help would be great!

Is it better for fuel mileage to run it more advanced? I'm thinking that when I didn't have it base timed and it was getting great mileage It must have been timed at about 14 BTDC and that would be like 10 BTDC base timed so maybe I need to advance more?