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This concept came from rocketry, but is very useful when applied to road (and other) transport. Payload ratio is defined as the ratio of the payload to the remainder of the vehicle. Also see Weight-to-power ratio.

Arithmetic The payload ratio is calculated by dividing the weight of the payload - the passenger - by the weight of the vehicle. In the case of the 1000kg car, 70/1000=0.07, which converts to 7%. The bicycle calculation, 70/20=350%. A big four-wheel-drive (4WD) or sports-utility-vehicle (SUV) weighs about 2000kg, so 70/2000=3.5%.   Increasing the payload-ratio If Mum, Dad and the kids total 150kg, and the scooter weighs 70kg then the payload-ratio is 150/70=2.14 or 214%

A person weighing 70kg travelling in a car weighing 1000kg represents a payload ratio (PR) of 7% (see the box on the right for the arithmetic). This is a very inefficient method of transport. A 70kg person travelling on a 20kg bicycle has a PR of 350%; much more efficient. The same 70kg person in a 2 tonne vehicle has a PR of only 3.5%!

If we are to reduce greenhouse gas production rates it is essential that we stop travelling with hundreds of kilograms of unnecessary steel and plastic. The payload ratios of our vehicles must be increased.

In Australia, and probably in many other countries, ultra-light motor vehicles cannot be legally driven on the road because they would not pass crash tests. Yet motor cycles can legally be ridden and are not subject to crash tests!

It must be quite possible to build an ultra-light vehicle suitable for one passenger and weighing around 200kg. A two passenger version could, I suspect, be built at a weight of no more than 300kg. Such vehicles could have, by present standards, enormous efficiency in terms of kilometres travelled per litre of fuel consumed (and per gram of greenhouse carbon dioxide released). However, essential while such vehicles are if we are to seriously reduce our greenhouse gas production, they would, under present road rules, be illegal!

I suspect that the average PR of vehicles on the roads will greatly increase as fuel prices rise because there will be a strong economic incentive. People are always moved more by the prospect of saving a few dollars than they are by the prospect of doing their little bit to save the Earth.

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Increase your payload ratio, tow a trailer rather than use a bigger car

Arithmetic The car: payload, 70+70+20+50=210kg; remainder of vehicle=1000kg; payload ratio 210/1000=0.21=21%. The car and trailer: payload, 210kg+500kg=710kg; remainder of vehicle, 1000kg+200kg=1200kg; payload ratio=710/1200=0.59=59%.   A trailer can increase your payload ratio

The car in the photo at the right weighs about one tonne (1000kg). It's typical payload - two passengers, a dog, and 50kg of luggage - gives a payload ratio of 21%.

This can be increased by towing a half tonne load on a trailer. The trailer itself weighs about 200kg. The new payload ratio becomes 59%.

Of course this is not saying that you should tow a trailer when you don't need to just to increase your payload ratio. It demonstrates that the typical modern transport, the motor car, is normally a very inefficient way of travelling. If you buy a bigger car so that you can carry more, your fuel consumption increases and the amount of carbon dioxide you are responsible for dumping into the atmosphere increases.

It is a little more efficient to use a smaller car and tow a trailer once in a while.

Links

Fuel-Efficient Vehicles - Now, Payload ratio

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