Can I get better fuel mileage

[Fergie]

2012 Pete 386 Cummins 2250 that has been demandated by Unilever.

I've put 350 miles since Unilever handiwork. Before I was getting 5.9mpg both in dash and on paper. Now I'm getting 7.8 in dash haven't had a chance to check on paper yet but can tell it by the gauge and I'm thinking it's probably more in line with 8.2.

I haul grain locally for farmers so my typical trip is go a couple miles of gravel, turn and go 4 miles then onto highway where I go about 20 miles and then usually have to drive through town to get to the elevator. Average round trip is 90 miles. I've reduced speed to 57 empty and 58 loaded. 58 loaded I'm in last gear(9) and 1250 rpm. Got super singles on trailer.

So after all the above I'm on the right track, just curious if you think I'll be able to get more out of it. I'm thinking not cause of the frequent turns. Just by doing this I'll be saving $7k a year. Thinking about running super singles on the tractor from march-november to pick up a couple tenths. Take them off for winter.

One different question if I could ask, has anyone converted their transmission? I've got a 9 speed that can be converted to 13. Cummins wants $190 for the kit. All the kit appears to be is a shifter knob, airlines, and fittings. I can get a knob off eBay for $40. If anyone has done it could you walk me through it? Thanks in advance.