Indirect TPMS


Direct and indirect tire pressure monitoring systems (TPMS) began to appear in cars over a decade ago. TPMS penetration is rising around the world as more governments make them mandatory on new cars.

How do the two types under adoption - Direct TPMS and Indirect TPMS - differ?

Indirect TPMS relies on cars' anti-lock braking system (ABS). Since tire inflation levels affect tire rotation, indirect TPMS gauges the differences in wheel rotation to detect under-inflation. Unfortunately, it cannot detect all four tires being under-inflated and is prone to false alarms, for example when a long curve in the road causes the outside wheels to rotate more than the inside wheels.

The more common direct TPMS installs a monitor directly in the valve of each tire and lights up a dashboard alert when the tire pressure is 25% or more below the recommended level. But direct TPMS has issues too.

First, each monitor requires a battery - a hazardous waste material. Three billion tires manufactured in the world annually means that if direct TPMS gained worldwide acceptance, it would create 3 billion more hazardous waste materials every year.
Second, while it warns of extreme, accident-prone under-inflation, direct TPMS does nothing for the 1% to 24% under-inflation that is more characteristic of most under-inflated tires, and therefore most of the extra carbon dioxide emissions.

Third, since direct TPMS will cost hundreds of dollars to maintain over the course of its life, adoption by cars already on the road will be limited, which means that even if every country in the world required them for new cars, full adoption will take 15-20 years for the pre-2008 model cars to be retired from the road.

The US law that gave rise to the direct TPMS originated from the US road safety agency whose goal was to reduce the number of accidents caused by tire failures, not to protect the environment. While direct TPMS may achieve the road safety objective, neither direct nor indirect TPMS was designed to solve the environmental issue at hand.


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Under-inflated tires