This sort of mpg claim (and a very good one, too) is better substantiated by running the truck over a certified scale
http://catscale.findlocation.com/
both empty and full to achieve payload carried (it's a moving truck after all), and an estimate of cubic foot capacity actually used versus U-Haul published figures.
I could tell you that I rented a U-Haul trailer and achieved 19 mpg (on two occasions; roundtrip same road over 1,200-miles) but it may mean more than I was grossing 12k with 350 c/f capacity filled and a payload of over 3k where the trailer weighed above 2k and the truck above 7k when both were empty.
Or, that it cost me W-dollars to move X-cubic feet of goods at Y-weight . . and that my fuel savings reduced the cost per pound or c/f move by Z-percent.
Changes in travel speed have to be factored against an overnight stay for slower travel (and safety, not factorable in $$ is in driving daylight hours only, with stops every 2-hours, with extended breaks every 4-hours, etc), so is fuel savings offsetting.
And, as the IRS allows 24-cpm deductible moving costs, how is the fuel cost savings represented against the actual per-mile cost of the truck?
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