Ok, so the air inside itself is also traveling at 75 mph, which is why if someone smokes inside the car, and the window is cracked, it all goes whoosh (very scientific term, whoosh) out the window like airplane with a crack.
I think I understand. How simply defined. Thank you. That has bugged me for years.
More or less.
And if you hold a ball in your hand and throw it straight up, it falls right back into your hand.
You and the ball are traveling at 75 mph. In the same direction, horizontally. So you see the ball go straight up vertically and straight down. You see no horizontal movement because you are sharing the same horizontal movement as the ball.
An outside observer, standing by the road would see the ball travel in an arc. He is not sharing your horizontal movement. From his perspective, you move straight horizontally, while the ball moves both horizontally and vertically. In an arc.