Overdrive & 4th gear normal highway driving

Hi can somebody explain the difference between overdrive & not driving in overdrive in 4th gear on the highway etc? Thanks. :)

Hi can somebody explain the difference between overdrive & not driving in overdrive in 4th gear on the highway etc? Thanks. :)

There shouldn't be a difference - 4th gear and overdrive are the same thing. I'm sure that someone will correct me if I'm wrong, but I don't think so.