which drive gear should i use?

so my 92 explorer has two drives apparently. one that that says d and one with a box around the d. i use the one with the box around it, im not even sure what the other one is (overdrive?) it seems to run at a higher rpm. can anyone tell me the difference? i dont have a manual so i cant check. any mpg difference or anything? thanks. i know stupid question, at least someone should know then.

For normal driving, you want to use Overdrive, which is the D with the box "over" it. Drive, which is designated with the D by itself, is basically just intended for driving with a heavy load in hilly terrain where the transmission would be hunting around if left in Overdrive.

-Rod