How high to rev before shifting in each gear?

Originally Posted by eurojjj19

I did see those numbers in the manual, they are:

1st to 2nd - 15mph
2nd to 3rd - 25 mph
3rd to 4th - 40 mph
4th to 5th - 45 mph
5th to 6th - 50 mph

But i couldnt wrap my head around the point of having to shift in 5mph increments (4-->5-->6). That just seems to make 4th and 5th gear pretty worhtless imo. Im not trying to argue, just explaining my thought process.