One driver was trucking more than 200 jars of jam around in the back; most were broken and the weight was dragging the car down and leading to higher fuel consumption.
Cost is not the only issue; carting around too much personal property can lead to some inconvenience if your car is forced off the road for repairs.
One driver even lost his false leg for a few days when his car was towed away from an accident scene.
Other finds in cars include secret documents left by a diplomat; mouldy shopping that had defrosted, plus cash and jewellery.
Crooks left behind a complete stolen cash machine in a car they dumped.
Insurance industry statistics also reveal one in five drivers have had an accident resulting from too much rubbish in their cars.
In a survey, only half of drivers clean the inside of the car once or more a year; while 8% confess they never bother.
“We all carry around items that we do not need in the car all the time. Not only could it be an inconvenience if you were to break down or have an accident, the extra weight you are carrying around will use more fuel. With petrol prices hitting an all-time high, this will make every journey more expensive,” said a spokesman for Churchill car insurance.
“Clutter can also be dangerous if it flies around and hits someone if the vehicle suddenly brakes. Rubbish on the floor such as bottles could roll under pedals, so make sure everything is secure that needs to be in the vehicle.”
The revelations are part of a push by car insurance companies to encourage drivers to keep their cars cleaner to cut down accidents.