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Just the same as with buckets of water. Two identical tyres would end up at half pressure. Another, donor, tyre of twice the volume would produce 2/3 pressure in both. And so on .
Hi Neilthe results may not be what you expect; although the pressure might equalise between the two tyres, the volume might not. Have a look at this "two balloons" kitchen science experiment we did on the show recently:http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/balloons-on-a-tube/Chris
Except that balloons are very flexible and a lot of energy is stored in the stretched rubber (the volume / pressure relationship is non-linear) Motor tyres are much more rigid. If the 'empty' tyre is not actually squashed flat then my simple equation would work. I'm not sure how much air would be squeezed out of an uninflated tyre if the car were to be lowered onto it.This would be an added factor. There is only 2Bar of excess pressure in a road tyre so the simple model may not give the right answer.Air cylinders used in diving work more 'ideally' because the pressures are higher and the 'envelope' can be considered as rigid - In the Navy divers used to decant air from one cylinder to another as part of their dive plan. (More reliable than using a contents gauge).
Gases behave more nearly ideally at low pressures and, at low pressures the deformation of the container will be smaller.If you jack up your car and sit on the other acr when you connect the tyres you will pump slightly more than half the air (assuming identical tyres and similar cars)
While you're at it, you may as well borrow your neighbor's petrol as well 
Quote from: sophiecentaur on 10/10/2008 15:08:09Just the same as with buckets of water. Two identical tyres would end up at half pressure. Another, donor, tyre of twice the volume would produce 2/3 pressure in both. And so on . THANK EWE very much our esteemed Silver Surfer Sophiecentaur !