[JohnAh]

A rough calculation told me that even if I have a quite big house in Sweden where winters may be cold I still spend about two or three times more money on fuel for my car than heating the house. Knowing that my car wastes 2/3 of the fuel as heat feels rather annoying this time of the year.

My experiences from many years with under-motorized cars says that weight reduction is not always beneficial. The average travel time have not differed notably between the number of passengers for example. A high weight may even be better when driving through a landscape with many smaller hills and slopes. A higher weight may act as a flywheel smoothing out the speed once it has been reached.

So, how about installing at heat accumulator to store some of the lossy heat from the engine and use this "free" energy for household heating? I know there are some saline solutions that can store way more energy than plain water but they may be expensive? Plain water is cheap and not the slightest harmful to the enviroment. I don´t remember how I got the numbers but I once calculated that an ordinary oil barrel of 200 liters should be able to store the heat generated from some 10´s of kilometers of driving.