I am the same as you, so I took my Senseair S8 for a ride once. It takes almost no time for two people to push it to 2000 ppm (which is where its accuracy start

I am the same as you, so I took my Senseair S8 for a ride once. It takes almost no time for two people to push it to 2000 ppm (which is where its accuracy starts to go out of whack), and maybe half an hour to 10000 ppm (which is the absolute upper level it can measure, so the real level may be even higher). I've used a cheap car though, and it shuts off recirculation using a timer. Maybe it's different in more advanced models. The sensor may be inaccurate, although it's one of the better ones if you're not willing to pay hundreds of dollars. After living for many years at similar levels of CO₂, I don't really feel them at all, so it's hard to (dis-)confirm these findings.