I have noticed that when displaying the link speed, SNMP mastering reports 11Mbps, RouterOS mastering reports 10Mbps. When specifying the link speed, it has no effect on that display, i.e. it reports 11 or 10Mbps no matter what.
HOWEVER, it drastically improves the bandwidth reporting for the longer periods. Almost any link that doesnt specify speed will have a yearly maximum average bandwidth of maximum 400k on the chart. When specifying, e.g. 24000000 for 24Mbps operating bitrate, the smoothing over the long periods is drastically reduced, and more reasonable bitrates are produced for especially year and month, and more accurate on all other time periods.
This has not always been the case. Using Dude 3.4 on x86 based RouterOS 3.23 on a Dell 750 with 2.8GHz proc w/512MB RAM...
Dont know how the actual bitrate can be determined on the fly, and maintain bandwidth graphs that are accurate. It seems that just specifying a link speed, like 54000000 for an '802.11a' type connection, and I specified 'unknown' for link type, reports the same improved graphs no matter what speed is indicated. I will monitor this over time to see if there is any difference over a longer period of time.