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.
Exhaust gas temperature gauge (or pyrometer): This gauge works in conjunction with the turbo boost gauge and will inform you if the exhaust gas temperature is too high or if there is a boost leak. If you’re lugging the engine, the fuel filter is dirty or there is a power problem, you’ll need to know the differences in the boost and pyrometer gauges.