I run a ratio of 2.0588 sprocket set up (divide rear sprocket tooth count by front sprocket tooth count) and I weigh 175 , and the bike tops to 70 indicated in 4th , then I switch to 5th as an overdrive gear for good milage on the highway

but 5th has NO UMPH but its right at its max rpm/hp ratio = about 10hp around 5500-6500 rpm , as push rod motors valves start to float much past that and hp drops sharply

I think I shift to 5th about 4500 or 5000 rpms around 65-70 and have "maxed" the motor at 80 mph when my valves began to float and no more rpms could be attained , but thats rare usually holds 70 + or - 5 indicated depending on wind direction and ability to draft

if your looking for slightly more gettyup from 5th than just holding 70 indicated , go for a 2.187 or 2.333 set up

BUT with every increase in ratio , you WILL lower attainable top speed and increase rpm's at highway speeds , a 2.333 for example could prob reach 70ish but never go any faster while screaming max rpms