if you want to fix your tach, get one of these.
I i believe you will need the 2/3 multiplyer for a 6 cylinder engine and a 4 cylinder tach. I think that's what i got for my eg33 swap. or just spend the extra 10 bux and get adjustable one.
alteast with the tach right you can do some maths to figure out how fast your going until you get the speedo sorted out.