How to weigh a car using a tape measure and a tyre pressure gauge

There are two pretty good ways to weigh a car:

1. Drive it to the nearest public weighbridge and wait your turn.
2. Look up the manual under General Specifications, ‘kerb weight’

But let's suppose the nearest public weighbridge is fifty miles away and you've lost the manual (and temporarily forgotten how to use public libraries and the Internet) and the burning urge to weigh your car just won't go away- what can you do about it?

The good news is, you need hardly any equipment. All you need, for a reasonably accurate result is:

• a hand-held tyre pressure gauge which can be analogue or digital
• a retractable steel measuring tape
• a calculator, pencil and paper, or a good head for mental arithmetic

The method

• Park the car on some clean, level concrete
• Observe that the car is held up by its four tyres (!)
• Measure the width of the tread of one tyre: e.g. 6 inches
• Measure the length of tread in contact with the ground: e.g. 7 inches
• Work out the area of tread touching the ground: e.g. 6 x 7 = 42 sq ins
• Now use the gauge to measure the tyre pressure: e.g. 25 psi (pounds per square inch)
• This tyre is therefore supporting 42 x 25 = 1,050 pounds
• Repeat for the remaining three tyres and add the four results together for the total weight.
• Front tyres carry more weight than rear: e.g. 1,050 + 1,050 + 900 + 900 = 3,900 pounds

If you are in a hurry, just measure one front tyre and the rear tyre on the same side. Sum them together and multiply by two. This works because cars tend to be left/right symmetrical.

The method can be used on a wide range of vehicles, such as:

• Cars, pickups, minibuses, buses, vans, lorries, old Bullnose Mercedes trucks, articulated trucks including effing-great eighteen-wheelers that shouldn’t be allowed on the roads, garbage collectors, tankers (full or empty), concrete pumps and aeroplanes, but not railway engines, steam-rollers or seaplanes with their floats down.

To weigh your seaplane...

You will need slightly different equipment. Forget the pressure gauge. The thing has floats, not tyres. The steel tape measure will still come in handy though:

• Procure an extremely large displacement tank and fill it with water.
• Using a crane, gently lower your stupid seaplane into the tank.
• Collect the displaced water in another smaller tank.
• Measure the volume of displaced water. (This is where your steel tape measure comes in).
• Apply Archimedes Principle to obtain the weight of the blasted seaplane, shouting Eureka at the appropriate moment, if you like.

Motorbikes are tricky too...

because they fall over. To weigh your motorbike, if you really must: 

• Get a set of bathroom scales.
• Park your bike with the kickstand resting on the scales.
• Measure the area of both tyre ‘footprints’, as above.
• Measure the tyre pressures.
• Calculate the weights on front and rear tyres as above.
• Add these to the reading on the bathroom scales.
• Job done.

While you’re hanging around like a lemon with the bathroom scales, you can weigh any convenient bicycle by the simple method of:

• standing on the scales and noting your weight.
• picking up the bicycle and noting the combined weight.
• subtracting your weight from the combined weight.

This method can also be used to weigh dogs, cats, sacks of wheat, sea turtles and children, but not hamsters or postage stamps, because the difference is too small to register on bathroom scales.

To weigh your hamster, use the kitchen scales instead. Can I go now?