If you want to use items to track profitability, you should set it up as non-inventory item, if you think you might sell the tractors at some point. If not, set it up as a service item. Check the box next to “This item is used in an assembly…” and you’ll be able to enter both the revenue and expense accounts. You should probably use a cost of goods sold account for your expense account. Make sure to use the Items tab instead of the Expenses tab on all your purchase transactions (i.e., Enter Bills or Write Checks). Then use an Item Profitability report (Reports > Jobs, Time & Mileage) to evaluate the profitability.

However, you might find it easier to setup a class called tractors. This will also allow you to use more than one revenue and expense account. If you don’t have classes activated yet, go to Edit > Preferences > Accounting > Company Preferences and check the box next to Use class Tracking (you may need to be logged in as Admin to do this).  Make sure to enter the class on all your revenue and expense transactions related to your tractors.  Then you can use the Profit & Loss by Class report (Reports > Company & Financial) to evaluate profitability.