Every mile counts when you're averaging 10 mpg, so I'd say that the first mod should be made in the office, tweaking the routes.
The delivery routes probably aren't going to be optimal, so tweaking them daily to cut the drive time down will bring an immediate gain. There is always room for improvement in a route, and with the holiday season here that will reap a great benefit when they need it most.
With 21 trucks, a 10% overall increase in efficiency of the routes would immediately be the equivalent of having one of the drivers working for free every day.
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