Aftermarket Dash and inaccurate ECM Speed

[Royal-T-Ltd]

Okay - i have an issue that's been bugging me for a little while... i have an aftermarket dash and i use GPS signal for my speed [which is also verified by phone and its GPS] my problem is when looking at logs – my ECM sees a totally different speed.... its usually off roughly +10mph...

Anyone else have this issue and solved it... is it a bad VSS or something worn on the trans itself?

TIA
aaron