# How much does car boot/trunk weight increase petrol costs?

1. Sep 20, 2013

### mtallyn

I often leave my golf clubs in the car rather than bothering to take them out I just wondered how much extra this is costing me. I have an Hyundai i30 ps109, ~1.2-1.3 tonnes and the clubs are about 10kg but more than happy for average car specs/approximations to be used. Can anyone do the maths? I'd be really curious to know.

2. Sep 20, 2013

### MrAnchovy

You'd be lucky to get about 0.2% saving around town, less than half that on the motorway/autoroute/bahn/strada/pista etc. (source). Note that this is probably less than the weight of a quarter of a tank of fuel, so you would save more by only filling up to half way when refuelling.

3. Sep 20, 2013

### AlephZero

There are (at least) two separate parts to this.

First, every time you accelerate, the extra mass acquires kinetic energy ($mv^2/2$), and unless you car has a regenerative braking system, your brakes convert that energy into heat when you slow down.

Second, unless you adjust the tire pressures, extra weight increases the rolling resistance of the tires.

An extra 10kg weight probably won't make much difference, though.

4. Sep 21, 2013

### Baluncore

Thirdly; If you play golf once per week, but drive to work every day over a 1000 foot ridge. Then the mass must be raised 1000 ft, twice each day. PE = m * g * h

5. Sep 22, 2013

### skeptic2

There are three factors (maybe more) that affect gas mileage.

The first is wind resistance. This could be the biggest factor but the golf clubs won't affect it.

The second is the friction generated by the engine, air conditioning, bearings etc. The clubs won't appreciably affect this either.

The third is as has been mentioned already, the increased energy expended to accelerate the extra mass and the increased rolling resistance. This factor is probably proportional to the mass of the car, thus the ratio of the mass of the car without the clubs to the mass with the clubs would be the multiplier for this third factor. What is left is to determine what proportion of the total is the third factor.