I just recently had 373 installed in my automatic. which i love. I have not as of yet got a tuner to change the stuff that goes with it( speedo...ect) the shift pattern feels good, is there really any need to get a tuner other then the obvious speedo correction and is there any damage that could be done without a tuner correction.
The only downside to your speedo being 5 MPH off is you will rack up a few artificial miles on her...just remember at the end of the day to put it up on blocks and run it in reverse and they will disappear