hey mate yeah i know what your problem is. I had the same issue, i had an rb25det put in my old GTS and my speedo was wrong untill i put in the gtst speedo, so i suggest the speedo if your car is a GTST one and you need to find the GTS one again.
what are the other options i will be needing to make one myself soon, rumors you can use an SR sender?
I have heard you can use the Navara sender too can you modify that to be correct?
Seen the post on here modifying the VL turbo one..many people done that?