2006 Lincoln Navigator overdrive

Should I drive my 06 navigator on overdrive, or drive?

Not sure what you are asking. Assuming this is automatic trans you put selector in drive and at the proper time the trans will shift to overdrive on its own.

Overdrive gets you better fuel economy. If you hear the transmission “searching” and shifting a lot at high speeds (such as going up a mountain on the highway), you can turn off the overdrive to regular drive if the sound of the shifting back and forth bothers you

Fun fact, the transmission in the 2006 Navigator actually has two overdrive gears, 5th is a . 87 ratio and 6th is a .69. Under normal driving you should use overdrive. The only times where you would want to use drive would be if you were towing or perhaps descending a long grade.

You’re getting worthy opinions here. But what does the carmaker say about it? Read it in the owners manual.