What could cause odometer to read so much lower than GPS?

[cib24]

I calibrated my speedo this weekend and it is now only about 1-2mph too fast at 70 vs 7-9 mph off before. I also decided to check the trip computer compared to my GPS on my phone since I always knew it was a bit off and counted too fast. However, today after a bit of a drive it was reading much lower than it should have:

GPS - 40 miles

Trip - 32 miles

It can't be working in kms because then the number would be higher than 40 (about 64).

Has anyone had this kind of issue before? I'm not sure how it went from reading slightly fast to this much slower.

How do you correct the trip computer/odometer so it's accurate?

[clive]

Notoriously inaccurate from the factory is the answer, my UK spec FD only read correctly against a sat nav when I fitted 19" wheels on the back !!!!!!!!!!!!!!

[cib24]

I'm thinking it might have to do with the speedo converter on the car that was put on by Torque GT. I wouldn't know where to look for it though but I find it so odd that it used to read just slightly fast, like 32.6 vs 32 but now it's just way off.

Actually, is the trip computer related to the speedo screw? Prior to adjusting my speedo this weekend it was off by 7-9 mph. I wonder if there is a correlation?

[cib24]

I found the converter and unplugged it (Speed M8 Pro) but unfortunately that cause the speedo and odometer to read absolutely nothing so I had to plug it back in since I'm not keen on attempting to restore whatever wires have been spliced under the electrical tape and messing things. So now I've taken the cluster off again and attempted to speed up the speedo back to 7-9 mph to fast so it calculates correctly. I'll test drive it today and see where I get.