It's a gear after fifth. More than likely it is a less than 1 to 1 ratio, perhaps fifth is also less than 1 to 1.
Find the gear ratios in the transmission and find the ratios for each gear.
Just "slam the gears" and "mash the gas", 1 through 6.
I have driven manuals for years, most lately five speed.
I have no idea how that 6th gear fits into the mix, so I'm asking. Is it a "cruising" one for after #5, or is it used as a "step" between lower gears?