As far as I know, modern trucks tracks them by computer now.

Engine hours are the hours it’s running. Probably by oil pressure or something that can only be started when the engine is running.
Idle hours are just that, hours tracked by the computer idling, no movement.

they won’t be the same for any two trucks, unless they are identical in their operations. Idle hours are going to be much higher in a service truck, such as an ambulance that starts at the beginning of a shift, and sits idling all day and night until the shift is done, as opposed to a truck that runs the highway all day.

engine hours are the time it’s running, moving or not. Idle time is just the time it’s idling.

The higher the idle time, the more it Sat around and idled.

example.
if a truck has 100k miles, and 5000 engine hours, 2500 hrs idle time, means it was averaging 20 miles an hour over the life of the truck, and sat around idling a lot.
If a truck has 200k miles, and 2500 engine hours, 350 hrs idle time, means it was averaging 80 miles an hour over the life of the truck.

The first truck was probably a service truck in a city, that sat around idling a lot.
the sencond a highway truck.

 

The way it was explained to me by the service tech was this:
200,000 miles at 2500 engine hours with 350 idle= 2150 hours moving. Engine hours is hours moving and idle. So a little over 90 mph average.
At least that is what I understood he was saying.

 

I think it's confusing people who are trying to convert "idle" hours to "miles" to somehow assess the life of the motor. The Engine Oil Life monitor is said to tell you when you need an oil change. So if you "idled" for 2500hrs to see the Change Oil Soon" message it's NOT saying your motor incurred the equivalent of driving (moving from point A to point B) over 5,000 miles. Since there is a Scheduled Maintenance Guide in the Owner's Manual you will see a reference to using hours for servicing intervals for some maintenance actions.

It is what it is: Idle hours and Engine/Motor hours.