After working with owner-operators for the last 37 years, I’ve concluded there are at least nine items that struggling 
owner-ops usually are missing:
Turbo boost gauge: Driving without a turbo boost or manifold pressure gauge is like driving blind. The boost gauge will inform you when you have a leak in the charge air system, a failing turbo, clogged fuel lines, a faulty ECM, dirty air filter(s), a dirty fuel filter or are using too much throttle on the level, resulting in a fuel mileage loss.