OBC readout

I finally got my OBC to work and wondered if ther is a way to make it readout current speed / current MPG instead of average ??? And is there a way to calibrate or check speedo, I used to work at a place many years ago that would hook a D.C. to the speedo and at various voltages it would read different speeds. Mine is off by about 5 mph (radar gun) but at 50 mph on the speedo and OBC the gun says 45 mph. I plan to install taller tires to make up the difference in the speedo but this wont correct the OBC reading.