Would removing the seats from my car save fuel?
Lionel asks: My new diesel secondhand car has removable seats. They are quite heavy, let’s 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 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 worthwhile leaving the seats at home?
Chris Smith put Lionel's question to Imperial College's Stuart Higgins...
Chris - Should he take the seats out or would driving round with them really not make any difference?
Stuart - It would make a difference but probably not enough to warrant taking the seats out and especially the weight is all down to different mechanisms, so different things can affect how much energy the car uses and, in particular, you tyres. About three to seven percent of your energy that’s burnt as fuel goes through the rolling resistance of the tyre.
Chris - That’s because you’re bending and flexing the rubber is it? Everytime it goes round it’s squeezing and then stretching a bit and that’s converting some kinetic energy into some heat, isn’t it?
Stuart - That’s exactly right. So that’s where some of that energy is being lost and as you apply more load, more weight to your vehicle, that deformation, that amount of flexing goes up and that starts to increase.
Chris - So that’s why you should pump the tyres up and make sure you have well inflated tyres because then they’re not bending as much; you’re not wasting as much fuel flexing and re-shaping your tyres?
Stuart - Very true but, at the same time, there are other factors you might want to consider such as: how your tyres grip the road and other safety aspects that are unrelated. Now this is very very back of the envelope - this is the physicists disclaimer here. I was looking at some data that was measured in the U.S. of how the weight of a car relates to what its miles per gallon efficiency is and it works out at something like per kilogram of weight something like a reduction of .02 miles per gallon. That’s very rough and I’ve got a lot of caveats on that. But, applied to this situation, we say three seats and about 10 kilos each, if you’re doing about 6,500 miles, which is the U.K. average over a year, it works out at about 2 gallons of fuel which is about 11 quid a year.
Chris - I was going to say the other thing that I find interesting is I never fill my car right up full of fuel unless I’m going on a long journey where I’m not going to stop because otherwise you think, the mass of the fuel in the car, that big tank of fuel weighs the equivalent of a person, doesn’t it? And you are, especially if you’re driving round town you are accelerating and then breaking and adding all that energy to that fuel for no reason.
Stuart - And that’s exactly right. So if you look at things that would make a bigger difference than say moving the seats out, actually, your driving style is a huge big part of that. How fast you accelerate and that action has a big impact on how much fuel you are going to use.