I've changed from 3.9 to 4.1 gears in my 88 vert. The speedometer seems to be reading a bit high. What else is needed to correct? Matching speedometer?

 

Borrow a GPS & make a chart of how far it's off.
Then leave it that way.
Then you can say: "Yes judge I suspected my speedometer was off, but not that much!"
Then he may let you off on that ticket if you get it calibrated.
Most metro areas have a shop that can fit a reduction gear on the tranny speedo output.

 

The change that you've made in your final drive ratio (i.e., 3.9 to 4.1) will actually cause your vehicle speed to decrease for a constant engine speed. You true vehicle speed will be 3.9/4.1 or 95% less than it was at the same rpm. For example, 60 mph with the 3.9 gears would now be 57 mph with the 4.1 gears at a constant rpm. IMO, your speedo error is really nothing to be concerned about.

Hot_Dog
90 RX7 GXL
02 Acura RSX-S

 

5% less, not 95%

 

if you want it to be right you need the speedo gear out of a tranny with the 4.1 rear end.

but as above with only a minor diff I probably wouldn't bother.