Be happy that you are only 3mph off.
I have been trying to figure out my problem for a while. I am 4mph off at 35mph (35=39mph) and 7mph off at 75mph (75=82mph). I also gain about 1/10 of a mile per mile.
It is a pain having to do math every time I drive. Odd to me that an all stock 93 Si would do this.
michael