is it possible to calculate rpm knowing speed

http://www.motogelnar.cz/manual/YAMAHA/DT125X_RE05Service_Manual.pdf
i would like to get confirmation of a computation i made: reading on the manual i saw the primary gear ratio is 3.227, the secondary is (for me) 3.8 because i have the 15 tooth pinion and let's take for example the 5^ gear that has a 0.957 ratio. so the ratio between the piston and the rear wheel is 3.227•3.8•0.957=11.735. for example if i go at 90 km/h in 5^ gear it means that i go at 90/60•1000=1500 meters/minute. i measured that the distance i do with 1 rear wheel's rotation is 1.94 meters so, 1500/1,94=773.2 rotations in a minute. now if i multiply: 773.2•11.735=9,073 rpm!