speed o gear

No you can't do that. You need to add into the circuit an electronic recalibrator for the speedo such as a "speedcal". How far off is the stock tach? Mine is out less than the with of the needle which I would estimate to be close enough for a tach that is marked only in 500 rpm increments.

off the top of my head its probably off by about 500 rpm+/-. there so many little electronic things wrong with my car since i bought it that im discovering :-(