Caddyman, I think CapriRacer is saying that the likelihood of failure is high enough that the tires should be replaced. Not all tires will fail, but in general, this rule of thumb means that you won’t experience a failure with a ton of water behind you if you follow the recommendation. And the recommendation may not apply to your specific tires (but it might). It’s like the timing belt. We replace them around 100,000 miles because the cost of replacing the engine is so high if they belt fails in use. You might have a blow-out driving down the street with no load. Then again, it might be on a highway with a full load of water.
JT,
Actually I was commenting on Caddyman’s description of what causes tires to fail. It is really about the amount and the properties of the rubber between the belts. Thtat’s why age is a factor - the rubber gets attacked by oxygen over time… It is a common misconception that it is about rubber to steel adhesion.
But you are right, it is best to replace tires before they fail. There is a whole list of potential things that could happen and some are quite severe!
I often buy used tires for my daily driver from serve yourself junk yards such as Pull A Part and can buy tire/wheel assembly for around $25. each regardless of size. Often the tires are only a year or two old, have nearly new tread and are already mounted and balanced on wheels that are direct fit to my car. I’ve also used many tires beyond the recommended 6 year replacement with no problems. I just replaced the original tires on my '97 in 2010. A couple of them still had about 1/4 of the tread life, but I wanted to replace the complete set since this is the car I use for long distance driving. We put the two good ones on the rear of my son’s car which is used on shorter trips and he finished wearing them out before buying a new set for his car.
That old water truck, there is 6500 pounds of water (800 gallons) on her back and the six tires supporting that weight have never complained. Highway? No…it sees only dirt roads, 30 mph max, 8 miles round trip…
At 80 mph on 3 ply radials, I would be a little more cautious …