Re: Speed Volume option on radio
 

ABS is only useful in inferring grossly under-inflated tires... Tires that are less than 50% of normal pressure. ABS measures the amount of grip/slip when accelerating/braking, since the contact with the ground changes drastically with pressure, but not the "tire circumference" overall. A tire on the rack with zero psi, unmounted, will measure the same as one mounted, and loaded, to within 0.3%. Tires are NOT balloons folks, ( if that were the case, you could fold up your spare and store it in the glove compartment). A change is real, but a VERY small one. Too small to be used for the purpose of air pressure measurement.

ABS based or "indirect" TPMS don't meet Federal standards, and it is unlawful to use any "indirect" method of tire pressure measurement on any vehicle intendeded for use in the U.S. manufactured after October 1, 2007.
Indirect methods cannot measure the "25% low warning" as mandated by law. Also, ABS only measured "slippage of right tire compared to left". It won't activate any warning at all if tires are low on both sides.

Owner's manuals are non-technical documents, and use the general language like "ABS measures wheel speed" which really means it measures wheel slippage... which in turn does mean one tire is spinning faster than the others, but not due to size... but due to traction ( or lack of ).

-John