Actually, no, Red had it (half) right. You're making a
huge assumption that everybody plays the same way as you. My starter cars have never ended up with high mileage (that I've put on myself, anyways). I trade up to a mid-level car pretty quickly. And time-wise, you could finish a ton of races in the Miata, in Beginner and Amateur Leagues, and still not touch the mileage someone will rack up on a lone run through the F1 Championship series. More mileage, and more total time invested.
Plus, most people don't only play the game for completion percentages. They'll take a car to a track just to see how fast they can push it. They could be competing in things like the old online (er, offline

) racing around here. There's literally endless possibilities. My highest-mileage car in GT4 is my Alfa 155 Touring Car. Because I've ran it in the DTM series dozens, upon dozens, upon dozens of times, for money to fund my buying sprees.
If the question was "which car have you owned the longest?", well... that's really "what was your first car?" worded differently. Red said you were in the wrong thread, and that's not entirely true; for you the Miata, or ITR, or RX-7 could've very well been your most-used cars. But you trying to say peoples answers aren't realistic (or that the smart first-car choice is a Miata) is just wrong. I didn't use the GT3 credit cheat, and guess what my first car was? The '98 M Coupe

.