Tare Mass - Empty weight of trailer ATM - Aggregate Trailer Mass - Total Maximum weight of trailer including payload. GTM - Gross Trailer Mass - ATM minus Towball Mass i.e. the weight on the wheels. Tow ball Mass - The weight on the tow hitch GVM - Gross Vehicle Mass - The weight of the vehicle plus tow ball mass. GCM - Gross Combination Weight - GVM + GTM.