I've seen a few of these 'rpm vs mph' threads recently in various places - every one seems to imply that it's a variable relationship that can be affected by how the engine is running...
It is not variable. Not on the same bike (whether you change the engine or not), not on different models of bike with the same spec and tyre wear and pressure.
It doesn't matter if your engine is running at absolute peak performance or whether it is running on one cylinder with no oil in - the rpm will ALWAYS be the same for a given wheel rotational speed in the same gear. It also doen't matter what fairing you have, what handlebars you have, how much you ate the night before or whether there are 1 or 7 people on the bike (see caveats at the end).
Basics....
The engine drives the gearbox input shaft through the clutch, this is a direct 1:1 drive.
Then the gearbox has a set of absolutely fixed ratios, let's use the 1.67:1 reduction of the K75 top gear for this.
The output from the gearbox is transmitted through the shaft to the final drive unit - this is again a direct 1:1 drive.
The final drive has another reduction - let's assume a 10/32 FD, or 3.2:1.
So, at 1000rpm engine speed:
Gearbox input shaft = 1000rpm
Gearbox output shaft/propshaft = 598.8rpm
FD output/rear wheel = 187.1rpm
This relationship cannot vary - 2k engine rpm = 374.2 wheel rpm / 7k engine rpm = 1309.7 wheel rpm. As I said above, it matters not a jot whether your engine is shoving out 371bhp running with nitrous or whether it's struggling to make 7bhp on one cylinder.
So now we get to speed... We have the wheel turning at 1309.7rpm (7k engine rpm in top) and a 130/70-17 lasertec tyre fitted, diameter 613mm/1925mm circumference. At this rpm you will be travelling 2521172.5mm every minute. That's ~94mph...
So, make a 'chart' off these figures.
1k rpm = 13.5mph
2k rpm = 27mph
3k rpm = 40.5mph
4k rpm = 54mph
5k rpm = 67.5mph
6k rpm = 81mph
This won't be 100% accurate - it will be affected by rounding errors plus the fact I'm working off a manufacturer published tyre specification, hardly a real-world measurement.
As always - caveats apply:
If the shaft splines are worn enough to skip or the clutch is slipping then you are inadvertantly altering one of the physical components in the same way as swapping out the FD for a different ratio.
Actual road speed (real speed, not what the speedo says) can be affected by the following:
Tyre wear - more wear = less speed
Tyre pressure - lower pressure = less speed
Tyre/wheel size - smaller tyre = less speed
Enough load to compress the tyre therefore affecting it's rolling diameter - more load = less speed
It's also worth noting that the tyre diameter can change with speed - the faster you go the bigger the tyre effectively is due to centrifugal force pushing the middle of the tyre away from the spindle/axle.
If anyone would like to dispute this (the theory that is, I'm open to the end result figures being slightly off) then I'd love to hear from you.
