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Both types of systems exist, but from what I've seen the indirect TPMS systems you're describing are becoming less popular. I'm somewhat familiar with Mazdas anyhow and that's the way it's going with them.

For 2017 the Mazda6 used an indirect system: https://www.mazdausa.com/static/manuals/2017/mazda6/contents...

For 2020 it's a direct system with in-tire sensors: https://www.mazdausa.com/static/manuals/2020/mazda6/contents...

This is true of cheaper cars, like my Honda. German cars like Audis and Porsches have direct TPMS that can actually tell you the exact tire pressure.

Most Audis have indirect systems, but they have direct TPMS as an optional extra. At least that's the case on the European market, unsure about the US market. Some premium Audis also have loose wheel detection, which is based on a similar technology as the indirect TPMS.

Oh, interesting. Well, my car at least has sensors—either that or I paid way too much for valve stems. :)