My speedo says 20mph when the car is off reads 20-30 while driving.

What is your question?

I have been told vss but am not sure. That seems to be the main problem. It registers between 20-30 mph while driving. Car runs great otherwise. Every once in awhile the check engine light will come on and the car will start jerking at about 3000 rpm If you accelerate fast if you go slow it will shift fine.