Clean out the boot
Do you really want to pay to drag around that festering rubbish in the boot?
Everything you carry in your car adds weight to it. The more it weighs the more fuel is required to drag it arround. Its as simple as that, so if you do not actually need it why carry it?
Weight (or excess weight) is more of an issue when starting or stopping the car. When the car is cruising a few 10′s of kilograms is really not going to make much difference. However when you are pulling away (accelerating) the extra weight will be a significant drag factor on the car and will use more fuel. Conversely, when you need to break the extra momentum of the car will result in more braking effort being required which will wear your brakes out quicker.
Of course the hard core hypermiller will no doubt have recontructed his car entirely of balsa wood and clingfilm and will drive in the nude in order to save extra weight. your natural sense of dignity will render this option unlikely so a more sensible middle ground will be required. Anyway – have you seen some of the american hypermillers? Lets just say they are carrying a good few pounds of excess luggage themselves.
Some things in a car are essential – you really must carry your first aid kit and warning triangle. A small toolkit and some spare bulbs might not be a bad idea as well. Cold weather gear in case of breakdown or being stranded in cold conditions but again – why carry that thermal stuff in the summer? It is only going to be stinky and damp by the time you do want it.
Spare wheels are a matter of some debate. If you are driving a car with big chunky wheels a skinny space saver might well save quite a bit of weight. A more extreme option is to do away with the the spare altogether and carry a can of tire weld which can cope with most small punctures. Such stuff will not cope with a full blow out or tear and some tire places really don’t like fixing punctured tires when they are full of tire weld gunk.