Product support liabilities include estimated future payments related to product warranties and deferred revenues on optional extended warranties and R&M contracts. The Company generally offers one year warranties covering most of its vehicles and related aftermarket parts. For vehicles equipped with engines manufactured by PACCAR, the Company generally offers two year warranties on the engine. Specific terms and conditions vary depending on the product and the country of sale. Optional extended warranty and R&M contracts can be purchased for periods which generally range up to five years. Warranty expenses and reserves are estimated and recorded at the time products or contracts are sold based on historical and current data and reasonable expectations for the future regarding the frequency and cost of warranty claims, net of recoveries. The Company periodically assesses the adequacy of its recorded liabilities and adjusts them as appropriate to reflect actual experience. Revenue from extended warranty and R&M contracts is deferred and recognized to income generally on a straight-line basis over the contract period. Warranty and R&M costs on these contracts are recognized as incurred.

Changes in product support liabilities are summarized as follows: