Sorry, but I don't think you set it up correctly.
Generally an 18 wheeler tractor is depreciated over 3 years instead of 5 like a smaller vehicle. You need to select "truck driver for over-the-road use" in the "type of vehicle" in the vehicle section to set it up correctly.
An 18 wheeler tractor cannot take the standard mileage rate due to it's vehicle weight.
The half year convention says that a half year depreciation is taken the first year placed in service for depreciation purposes and the last year for depreciation purposes.
Here are links to IRS Publications 946 (Depreciation) and 463 (vehicle expenses)
IRS Publication 946
IRS Publication 463
[Edited 03/30/2021|3:05 pm pst]
