You can discount the small errors at low speeds. But the consistent 7 MPH offset is a zero error. Do not change the gear in the tranny.
I would suggest contacting the gauge manufacturer and ask them for their comments. There very well may be a zero adjustment screw, and if there is one, its probably accessible thru a small hole on the back of the speedo "canister".
I've got Smith gauges in my SPF, and amazingly it is only off by 2 MPH at 130 MPH.
