I'd assume it has to do with higher RPMs, and more starting and stopping. Our White 2-105 uses a Perkins diesel. When we're plowing (The hardest job on a tractor), we're running around 1600-1700 RPM, and it's pretty consistant. You throttle down at the ends to turn around, but that's about it. An "hour" is measured at the engine's rated speed (Our White's is 2300 RPM), so if you run it at half that speed (1150 RPM) for an hour, it only registers as 1/2 hour on the meter.
My Audi cruises at 65MPH around 2600 RPM. I drive on the highway mostly, so I do 5000mi OCIs. But, in the cities, there is a lot more starting and stopping. 90% of my mileage is on the highway, so my car likely has it a little easier than one in the city that has varying RPMs, loads, and acceleration....
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