On a lot of old cars up to the 2000s, they had automatic shifters that typically went like this: P R N D 3 2 L.
what do the numbers mean? This is NOT the auto manual shifter on basically all gasoline cars today.
Also forgot to mention this but overdrive means that you drive at a lower RPM for increased gas mileage, correct? This is obsolete today, but it was common on a lot of Japanese cars up to the 2000s.
The numbers mean that your car can not go beyond that gear in the case if you didn’t want the car to get to the 4th gear.
Regarding your follow-on question about overdrive, you are essentially correct: it shifts into the highest gear and engine operates at a lower (or lowest) RPM thereby improving fuel efficiency.
Owner's manual describes them in great detail.
In 2, the car won't go past 2nd gear. In 3, it won't go past 3rd gear, and it will only go to 4th gear after you put it back into drive.
Todays cars have D and M or D and L. But a majority also will let you +- shift through the gears manually.
