Yes. The overhead is essentially down to overheads on the framing, IP and TCP overheads.
The usual maximum packet size is 1500 bytes (there are so-called "jumbo-packets", but they are only used for specialist purposes, like local storage over IP and require special support). However, within that 1500 byte packet there is a 20 byte IP packet header and a 32 byte TCP header. That gives you a payload of 1448 bytes. However, there is also an Ethernet framing overhead comprising another 38 bytes. So, for every 1448 bytes of data there are 1538 bytes sent over the link. That's 94.1% payload efficiency, which means for a 40mbps sync there is a maximum of 37.6mbps data throughput available.
The proportion is the same no matter what the sync speed. So an 80mbps will have a maximum TCP payload throughput of 75.2mbps.
*** edited as there was a slight mistake where I'd used 1440 as the payload size ***
Note that this is for IPV4. IPV6 has larger overheads.
Also, there is an overhead for "ack" packets in the opposite direction (About 4%, so downloading at 40mbps requires a minimum upload bandwidth of 1.6mbps).
UDP protocols have different overheads, but that's not normally used for speed tests.
Edited by TheEulerID (Wed 01-Apr-15 14:07:37)