I have hand calculated every tank over the first 8,000km (5,000miles) and the onboard computer is accurate within 0.1L/100km (0.2mpg) on every tank so far. I’m attributing this to the fact that the diesel fuel consumption just isn’t as volatile as the gas version and is more rangebound.
On my diesel, this truck rarely wants to burn worse than 12L/100km (20mpg) around town and I easily get 9.5L/100km on highway trips (25mpg) at 75mph+. Nova Scotia is hilly as well, no flat highways around here.
I don’t ever drive 55-60mph but any vehicle will get good mileage at that speed. One neat feature of the onboard computer is that rolling fuel economy. I have mine set at last 50 miles/80km and I can share that my best 50 mile highway mileage at 75mph+ is 7.9L/100km on flattish highway. That works out to 30mpg.