I wonder could that be the real speed? With manufacturing tolerances and other factors. I mean why print 55.80mhz on an oscillator or processor ..just round down to the nearest and print( I mean on the actual chip).. There is always a + or - % either way when it comes to electronics. Could be SysInfo is pretty accurate...with somethings.
I'm probably way off so forgive me it is early. 
Rich
If it's 28MHz 040, it'll have a 56.000 MHz Oscillator on your card. If that's what's printed on the oscillator then that's precisely what the speed will be, whatever sysinfo reports. Oscillators are precisely manufactured to extremely high tolerance levels. They are designed to be perfectly accurate.
Try WhichAmiga
http://aminet.net/package/util/moni/WhichAmiga which seemingly reports correct clock speed.
