well, here is my theory - it's about ~5% of your upload rate. This is approximately the TCP Ack overhead for this connection, and uT shows it. As I remember , this is usually due to the fact that you have your adv->net.calc_overhead set to true (as I do... ). This is a good thing (at least for my poor unbalanced 1:10 connection) since it makes uT refer to the limits you set in the true sense of the total traffic, not only the data. So, the side effect is that you also see this overhead in the numbers. You can try to reset your calc_overhead (if I'm right ) and this will seem to disappear. Actually it's still out there - and you can see it using an external speed measurement tool... I would suggest to the devs to maybe add another option - to control if to VIEW the speed with or w/o the overhead.