In theory the supply of used cars should be greater from part-exchanges and re-fleetings in those months, but in fact they take 2 - 4 weeks to feed through, moving the March p/xs to April/May, which are months when the market is strongest and prices are highest. Not so 2 - 4 weeks after September p/xs are taken in because the market tends to bottom out in late October/early November when prices are relatively at their lowest.