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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
Also see Weight-to-power ratio.
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.
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.