Presumably achieving higher than 45 mph results in the enginer revs going into the red arc?
If you are only getting 45 mph at full throttle and the engine rpm is staying in the green arc or slightly above it, then a 5 speed gearbox is not going to make any difference.
The gearbox just matches the speed of the roadwheels to the engine speed which is most efficient, giving greatest power at lowest specific fuel consumption and also matching engine torque to that required at the wheels. If you don't have sufficient engine power to overcome body drag (skin friction drag, form drag), frictional losses in the drivetrain etc then you won't get any increase in top speed - extra gear ratios may just help you to get to that top speed a bit sooner.
JD
April 2009
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