Just posted this in Members' Rides but it's pertinent here also.
If you got a JDM gauge cluster with your clip, or just bought the cluster to have the tach, you can convert the stock Japanese 180 KPH cluster to read in MPH with the addition of a resistor and some fiddling with a potentiometer on the back. The procedure is detailed here:
180 mph speedo how-to
Since I don't have soldering equipment or experience I asked a buddy (thanks Eric) to actually solder the resistor in for me. He did a great job and made it easily removable if necessary.
As nearly as I can tell the speedo seems pretty accurate. I pegged it on 60 and had my wife time our travel between two mile markers on the interstate. The time was within a second or two of a minute, which is pretty much as it should be. Supposedly the mod also converts the odometer but I haven't tried to measure it yet and it seems to turn over more quickly than it should.
FYI, there's a company called LS Automotive that sells 180 MPH speedos for S13 that actually say MPH on the face instead of KPH. I have no idea whether they've just done the same thing as above and added a new face, or if theirs is more accurate (they advertise that it's calibrated) but theirs is a minimum of $100 at their ebay store; mine cost a few cents for the resistor and a favor from a friend.
End result:
