I filled my fronts with "foam". I am not sure why it is called foam because it appears more like a silicon rubbery substance when cured. It is very heavy unlike foam. My rears are filled with windshield washer fluid I purchased from a local jobber in a 55 gallon barrel for less than a dollar per gallon.
As far as the disputes on this thread, one must ask, why use a highly poisonous product when other options are available for the very same price? Flat tires are very common. I personally have lost a cat to antifreeze. It was an ugly death to top it off. His eyes were rolling back and it survived close to a day before it finally died. Our vet said it only takes drops from him licking his paw.
I am not a die hard cat lover or environmentalist but please be respectful the environment in which we all must thrive.
Jeff