What limits my upload?

[#1]

To be honest it's just curiosity that prompts me to ask. It's not a big deal for me.
I'm on 40/20 FTTC and at the moment I get a reliable 37Mbps download with about 23ms latency. My upload is always the same every time I test it at about 6.4Mbps so given that I'm getting pretty much full speed on my download, what are the possible causes of my upload being so relatively low? 

[#2]

Your current upload sync speed is 7.5 so the results your getting are pretty much what I'd expect.
It will most likely be distance from your cabinet that's the reason as your speeds are within their estimates,

[#3]

So is there no relationship between achievable download and upload speeds?

[#4]

It is an interesting question longedge because I get approximately the same speeds as you.
I see some people that get around the same download speeds as me, but get approximately twice the upload speed I get (and I am not talking about people limited by the 40 Mbps plusnet profile, I am talking about people with a max attainable rate of about 38 Mbps).
I think it has a lot to do with the quality of the line (joints, copper vs aluminium, distance of course, proximity to other signals, crosstalk etc).
I think there are groups of frequencies allocated to upload and download so I think if the characteristics of your line means it is lossy at those frequencies, you will lose chunks of your upload speed.
So I suspect a lot of this is to do with the frequency response of your line (between modem and cabinet) at the VDSL frequencies used.
If you have a suitable modem (either unlocked or capable of giving you the right stats), you can see a graph of what tones (frequency groups) are in use on your line. I suspect that would reveal a lot with regard to upload speeds.
At present, I only have the locked ECI modem so cannot get any stats. I will be interested to see these tones on my line at some point, although there may not be much I can do to improve things.

[#5]

Quote from: jafreer

I think it has a lot to do with the quality of the line (joints, copper vs aluminium, distance of course, proximity to other signals, crosstalk etc).

I think that must be the reason that I lose out on upload speed. I've certainly had my share of problems with the ancient cables that run to my cab which is about 3/4 mile away. As I said initially just idle curiosity really as the speed I get fulfills my needs. I'm only glad that I get the download speed that I do (frantically touching wood).