[Lionel]

Lionel  asked the Naked Scientists:
   
My new diesel secondhand car has removable seats. They are quite heavy, lets say 10 kilos each.
The trouble is, I park on the road, often around 300 metres from my house, so the seats are a bit too heavy to cart around on foot. Which means an extra car journey to pick up the seats if I leave them at home to save fuel.

So I am wondering what sort of mileage would make it worth leaving the seats at home.  (I've often thought the same things about leaving things in the boot; like jump leads and lights.) Cars are pretty heavy things, so the weight of the seat is small in comparison. But what is the approximate relation to the increment of a car's weight and its consumption? Is it linear?

All the best,Lionel.





   
What do you think?

[vhfpmr]

The fuel you use can be split into broadly four categories: hill climbing, acceleration, overcoming friction and overcoming wind resistance.

Energy used in climbing a hill is directly proportional to the total mass of the vehicle, so if 10kg were 1% of the total weight you would save 1% of that part of the energy used to climb the hills, which in turn will depend on how hilly your home area is.
The same goes for acceleration, it's also proportional to mass, so the amount saved there will depend on how heavy footed you are.
Some of the friction (eg wheel bearings) will depend on the weight, other bits (eg engine bearings) won't.
Wind resistance of course won't depend on the weight at all, but it's non linear, and gets proportionally greater as you go faster. If you do mainly motorway driving the wind resistance will be dominant, and the other weight-related losses will be a minimal part of the total, but if you do a lot of slower stop-start town driving wind is less significant.

[alancalverd]

Use the European fuel consumption figure K liters per 100 km.  If the mass of the entire car plus fuel plus driver is M and the removed seats m, then over several journeys the fuel consumption will be  K (M-m)/M. This calculation is nontrivial for long-haul aircraft (you need enough fuel to complete the journey plus a diversion, but any excess fuel increases your takeoff weight which increases the fuel burn.....) and racing cars (less fuel per fill = faster between pit stops, but more pit stops).  However since the point of a passenger car is immediate and convenient travel over a wide variety of missions with various passenger loads, the most sensible approach is to leave all the seats in place unless you really have no friends, in which case remove them all, or buy a smaller car. Ultimately, a motorbike is quickest and cheapest, and also solves the problem of what to wear when travelling: a leather suit and a helmet will suffice for all occasions.