</font><font color="blue" class="small">( um, I used to work at an automotive shop and there's a reasons why u have to change your antifreeze. The chemicals break down. we used to be able to actually test what tempature it would freeze at, the older the antifreeze the warmer the tempature. So I would not recommend using used antifreeze. also if you get a puncture or slow leak around the rim that u don't notice right away and your neighbors dog comes over and licks it up and dies you wouldn't feel very good! <font color="red"> </font> )</font>
First, ethylene glycol does not break down over time. Quite the contrary, it is a very stable substance and can be recycled and used over and over again with the same level of freeze protection. It is the corrosion inhibitors that are added to the ethylene glycol which can break down due mostly to the heat of the engine environment. The fluid in my tires will not get hot.
Maybe your tests were not accurate because people tend to add straight water to their cooling systems when topping off. This is the only way to "weaken" the mixtures freeze protection.
As far as toxicity, yes it is toxic. If the radiator on your car leaks and the fluid gets licked up by a dog or cat, they will die also. I park my machine indoors for the most part, and have no tubes so a leak can be repaired in the field. A "slow leak that I don't notice right away" would not likely leave a puddle.
As with many chemicals, common sense and care should be taken. Spills should be cleaned up and not left for pets to get at.