JIMNLIN said: "571.110 S2.2.1 says "except as noted in S4.2.2.2, the sum of the maximum load ratings of the tires fited to an axle shall not be less than the GAWR of the axle system as specified on the vehicles certification required by 49 CFR part 567 (FMVSS placard reg). "
If this quote is correct, it would appear that it would be against the law for a person to install a "D" rated tire (or C or P or whatever lower rating) in the place of an "E" rated tire if the vehicle came equipped with "E" rated tires from the manufacturer.
I doubt that is the case. It is far more likely that the reg quoted applies to the vehicle assembler/manufacturer rather than to the end user. This would also account for the reason the vehicle assembler/manufacturer "under rates" the axles when compared to the ratings given by the actual axle manufacturer.
If the assembler/manufacturer rated a SRW rear axle at the full 10,800 lbs. the axle manufacturer gives it, they'd have to install (according to the Goodyear tire rating chart) at least 275/70R22.5 tires on it to match that axle rating! (5400 lbs per tire at 90 psi single)
That would appear to answer a long standing question regarding the reason behind the axle ratings matching twice the tire ratings --- which would give credence to the fact that what matters is the DOT rating on the tires/wheels and the accurate comparison of the axle weight rating to a mattress tag.
Thanks Jim! You finally posted something that makes sense out of something that has bothered me for a while. Why do they do it(under rate the axle)? Because the regs require them to in order to be in compliance with the tag that has to be there to show the vehicles capabilities AS IT LEFT THE FACTORY, not necessarily after upfitting with more highly rated tires, wheels, suspension components, etc.
For what it's worth, Jim, you're the last person on here I would have expected that info and its resulting claification to come from. Thanks again!
