Transfert Speed Calculation

Whats the best way to calculate transfer speed?
Currently i made a byte transfered counter, which is resetted everytime its read by an external module. External module is responsible for calculating the time interval between each read, but if I want to use the speed in another module i'm screwed..
Also thought of having a map with ticks since started as key and packetsize as value. This way i could get every packet size that happened in the last X seconds and calculate current speed but I think that cleanup would require too much cpu...