you can go 100 in 3 rd gear lol and if your flooring it to 100 its in 3 rd, and you'll never see 4 th kick in when its floored.
over drive is what it is, its just a helper gear for mpg. its not a real gear like 1,2, or 3.
and at the slightly higher rpm the engine runs more freely as it not under as much load. most engines dont hit the start of the rpm power range till 2500 rpm, thats where it will start to pull its best and use the least amount of fuel/throttle. going up hill that is lol
on flat ground or hills that OD holds ok, leave it in OD. you'll get the best mpg that way for sure.
