CPM is the total cost to operate your truck for each mile driven — loaded or empty. It combines every fixed and variable expense into one number that tells you exactly how much money you spend per mile.
Your CPM applies to every mile — including empty miles where you earn nothing. Break-even RPM adjusts for that. If you run 10,000 total miles but only 8,500 are loaded, your break-even RPM is your CPM multiplied by the ratio of total to loaded miles.
Every load you accept below your break-even RPM costs you money — you are paying to haul that freight. Your break-even RPM is the absolute minimum. Your target RPM should be at least $0.80–1.00 above it to build real profit margin.