Interesting question - complicated answer. The set up: For every tire, there is a load table where the relationship between load carrying capacity and inflation pressure is defined. There is also the max load and its corresponding pressure indicated. For Standard Load Passenger Car tires, the max load occurs at 35 psi (2.5 bar, 250 kPa for metric tires = 36.3 psi) regardless of the speed rating. (For reference: metric tires = those tires not following the American tire standards which are expressed in English units, but are essentially the same) Where the additional margin of safety comes in is that the H rated tire has more speed capability than the S or T rated tires even at those lower pressures. For practical purposes, higher speed rated tires can be made to perform EXACTLY like lower speed rated tires, because the things that control wear, traction, ride, and handling are independent of what controls speed capability - BUT - it is common for higher speed rated tires to be designed to handle better (and therefore ride worse), get better traction (but wear worse, and get worse fuel economy). The only thing that is always worse for higher speed rated tires (all other things being equal) is fuel economy - and that is only slightly worse - not enough to worry about! Other types of tires, such as XL, LT, ST, etc. behave similarly, but the details vary.Originally Posted by CressidaMany vehicle manufacturers list on their tire placard a recommendation of 10 to 12 lbs. lower than that 44 psi that you say the H rated tires are speed tested. If one runs an "H" rated with the vehicle recommended (e.g. 32 psi); does that mean there is an additional margin of safety, or can you help us learn what that difference might mean to the tire behavior or characteristics (if anything) ?Originally Posted by CapriRacerThe difference in max inflation pressure? T rated tires are speed tested at 35 psi, and H rated tires are tested at 44 psi, so it appears the max pressures are following. (A side note: There are some vehicles that call for unusually high inflation pressures and I wonder if those max pressures are reflecting that!)
Interesting, I would have assumed that the lower temperature rating would imply worse fuel economy. My logic being that the additional heat build up is proportional to rolling resistance as the materials flexed. Can you share, maybe in just general terms, what the mechanics are behind the decrease in fuel economy? Is it just the additional weight penalty for the stiffer construction?
Thanks for sharing your tire knowledge!!