My Z28 hits the 115 mph then the speedo needle starts going back while accelerating until it settles at 100mph. The same time my RPM is climbing to 5500 RPM. I know I am driving at close to 125mph while the speedo still at 100mph.

I slow down and it starts moving down, then tried again and it starts dropping at 115mph again while accelerating till it stops at 100mph.

This happened after swapping the 96LT1 engine and programming the PCM. The guy who tuned it put the stock ratio for A4 although I have 3.42 ratio, gonna go back to him again, although I think it is not the ratio thing.

Could it be a bad VSS or something else. Scanned the PCM with EZ link and no codes. It is OBDI.

Any help is appreciated.