I have seen the 18,500 blade tip speed as a suggested speed for blades up to 20 inch... as the blade gets longer than that the turbulence increases and causes additional problems... My largest deck runs a slightly lower BTS ... the 84 inch Land Pride running 3 blades 29 inches each, spin at 17,210 ft per min.
My 72 inch CTL with 3, a little over 24 inch blades, runs right at 19,000 fpm... In fine bladed grass I slow it down a bit for the 'perfect cut'.
So the 'Ideal' blade tip speed would appear to vary from grass to grass and blade length and deck design...
A 20 inch blade spun at 3,600 RPM is going to have right at 18,800 FPM... (that is going to be the average lawn mower)... Keep the blades sharp you could shed some RPM and still have a good cut... :smile:KennyV