top gear is 1:1, so the driveshaft speed will be the engine rpm.
if you need a gearing calculator to figure out what the effect the rear axle ratio and tire size will have on engine rpm, I like to use the unix nerd one I found years ago for BMW, easy to change, you might have to do an inch to mm conversion on your tire size if its different than the width in mm/aspect ratio/rim diameter standard though.
enter your tire diameter, enter rear gear ratio, enter rpm and use 4th gear, its 1:1, then select calculate speed. or, if you know speed, enter speed and calculate rpm.
http://www.unixnerd.demon.co.uk/revs.html
working backwards (entering speed and calculating rpm), you can see what the effect of a lower gear ratio will be.