Below is a list of mileage on my recent trip to and from Colorado on 32" M/T tires - Speed was between 65 and 80, with 70 likely about average and the A/C was running most all the time. The lower numbers were either climbing in elevation or higher speed and the higher numbers were either losing elevation or holding mostly to 65mph. Below 65 should yield better numbers but I was in a hurry. Base ignition timing is set at 7 degrees BTDC, but seeing the way the timing is controlled by the ECU, I'm not sure there's any benefit to setting it at the upper limit - maybe.... I'd have to reset it to 3 BTDC and see if the curve moves down by 4 degrees overall.

16.4
19.3
19.9
18.9
17.7
16.3
17.4
20.3
Total mileage/gal SC to CO = 2,109/116.4 = 18.1 mpg

19.1
18.6
18.9
17.3
15.3
17.3
17.5
18.5
Total mileage/gal from CO to SC = 2,472/138.6 = 17.8 mpg

Total mileage/gal round trip = 4,581/255.0 = 18.0 mpg