You've obviously not watched many episodes of (the original) 'Minder'! ;-)
The phrase clearly comes from the (alleged?) habit of people who're lacking in knowledge (or, more likely, just lacking in interest) of 'inspecting' a secondhand car by simple walking around it and kicking the tyres as a gesture of examining the vehicle.
An experienced car dealer will know that he's got a potential sale when a 'customer' insists on looking under the bonnet and under the car. If the 'customer' simply kicks the tyres he's either a complete fool or, far more likely, a complete time-waster.
Chris