4000 revs at 70 = 17.5mph per 1000rpm which is the accepted measure in the UK. 17.5 is quite low. Top gear is conventionally 1:1 in 4 speeders.
At 4.1:1 when engine revs are 1000, wheel revs are 243.90 at 17.5mph
At 3.7:1 the wheel revs would be 270.27, an increase of 26.37 at 1000revs, so 10.8% which means at 4000 revs you would see a 10.8% increase on road speed, so 77.57mph.
Any lower than 3.5:1 you would feel the difference on acceleration. A five speed box - assuming fifth was an overdrive ratio and not 1:1 should give a similar outcome. In this situation, ovedrive fifth ratio's will be typically 0.87.
Going up a tyre size will also help, but you need to consult a tyre size comparison site to establish circumferences, and the difference is smaller that you will get with either of the above changes.
All that is very fag packet by the way!!!
