I know the trade off with any gearing
taller diff/gear = acceleration is slower but lower revs for fixed speed
shorter = better acceleration, but you can run out of revs at terminal velocity
I asked, merely because I'm sick of sitting on 3kRPM at 100km, and wanted to knock it down a peg or 2. I thought that would be the easier option than trying to change the 5th gear ratio in the FS5W71C gearbox down to about .75 or .7 instead of .838.
It might be the easier option, but don't know about the cheaper option.
Speed is not the issue, it's comfortable cruising. Next fix might involve sound deadening foam just to shut the diff up for now.