Or “Overdrive.”
Top Gear is a reference to the “4th” Gear in the gear box.
The modern “fifth gear” was the ‘overdrive’, at the time the show was originally conceived. Even now ‘To be in Top’ can be a reference to either 4th or 5th gear, depending on the age of the speaker.
The Term “Top Gear” Also means: The Best “Stuff”.
The term “Fifth Gear” was taken by Channel Five, with an equally good programme.