Just got this on the road and am not getting the mileage I had been expecting, pulled around 23MPG on the first tank. It has been primarily flat hwy going 60MPH. What should I be expecting for mileage with this type of driving?

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300TD W124, Two VW TDI Passat Wagons,Cummins Ram 250, Kubota Tractor
23 cylinders sipping the sweet sauce of the soy bean