Originally Posted By: HTSS_TR
CapriRacer, I know you're a tire expert of this forum. Your opinions are well regarded by me and many others. But I disagree with you on several points:
This is not a myth, what I said was my experiences with different speed rated tires on 1994 Lexus LS400. The car is capable to sustain up to 150 mph with V rated tires which is original equipment and recommended by Lexus.
With lower speed rated tires, to be able to drive at fairly high speed in desert heat, I increase the cold tire pressure to 34 PSI while the recommended is 30 PSI at So Cal ambient temperature at that time was 75-80F and ambient temperature at Death Valley was 115-125F, the max pressure on sidewall of that tire was 44 PSI. Cruise control was set at 85MPH and one in a while for passing traffic I sped up to 100-110 MPH without much drama. Yes, it did not feel as stable as with V rated tires but the car did not feel as it was about to fly off highway either. I believe tire manufacture built in some margin for speed rating, an S rated (113 MPH) can sustain 115-120 MPH for several hours without blowing up if it did not overloaded.
This combination is dangerous: tire with lower speed rate + lower pressure + overloaded and higher sustain speed with high ambient temperature.
HTSS_TR,
Let me explain it this way:
This is all about risk.
There are 5 states that account for about 80% of the tire failures: AZ, CA, NV, TX and FL - pretty much in that order. A lot of this is population driven (meaning I would have expected other hot weather states to be in this mix, but I don't have enough information to do the statistics weighted by exposure. That is: the number of identical tires in the popluation)
Plus, these tire failures are almost exclusively on S and T rated tires.
Operating a tire at a lower load than the value on the sidewall and operating the tire at an increased tire pressure over the placard reduces the risk, but using tires with at least an H rating has an far. far greater effect.
As you point out, you haven't experienced a tire failure, but others have. The actual failure rates are low, but the consequences are pretty severe.
It is the severity of the consequences that is driving my statement. Clearly there are combinations where you will "not be a statistic", but there are things that can be done to greatly reduce the risk - like using a higher speed rated tire.
Put another way, if you operate your tires the way you described, there might be a 1 in 10,000 chance that you'll have a tire failure, but if you use an H rated tire the risk is virtually nil.