Just ran across this thread on the '05+ Tacoma board, and would appreciate your input regarding Nitto's technical bulletin.
http://www.toyotanation.com/forum/t200617.html
As I posted in that thread, I believe a vehicle's weight is supported by the air pressure within the tire, and not by the tire's sidewall (run-flats excluded). And that increasing load-range ratings are made possible by stiffer, heavier tires (with more and/or heavier plies), therefore enabling the tire to contain more air pressure -- and that increased air pressure allows the tire to support more weight. By this reasoning, when replacing a vehicle's P-rated tires with LT tires (say, LR-C or D) -- although the LTs have greater maximum load capacity (at greater pressure), you would still run the same inflation, because it's that air pressure which is supporting the load. According to the bulletin, however, my long-term thinking is all wrong.
The bulletin states that LTs require higher air pressures to carry equivalent loads. Of course, higher pressures will also give a stiffer ride. And they seem to contradict themselves: "
the ability of any tire to support a specific load is solely based on the inflation pressure within the tire" would indicate to me that if I replace my OEM P-metrics with LTs, I would keep the same pressure. That statement agrees with what I just wrote in the above paragraph. But then they follow that with "
size-for-size, LT-metric tires require higher air pressures to carry equivalent loads of P-metric tires". The two statements seem to be in direct contradiction.
Their example is of a P-metric at max load, and of course tires typically do not support their maximum, so pressures are usually set somewhere below maximum. I have never seen any chart indicating how much pressure to run for a given load, nor do people typically weigh each corner of their vehicle. Rather, we use the manufacturer's recommendation and adjust as needed based on wear, handling, comfort etc. If you switch to LTs, how would you know what pressures to run?
Also, their illustrations show the temperatures of LT tires at max inflation vs underinflated (we can only assume maximum load), but they fail to show P-metric tires at normal inflation vs LT tires at the same inflation (at normal load) -- which would be a most useful comparison for those of us considering LT replacements at OEM pressures.
According to their bulletin, it would seem that replacing P-metrics with LT-metrics (on light trucks/SUVs) for normal passenger use is not such a good idea, as the tires will run too hot if you keep the same inflation, or will ride stiffer if you set the pressure higher. And at any rate, you would never know what pressure to run. What am I missing here?
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