There's at least one other electronic box to correct the problem, but I
can't find my link to it's web page.
As I recall, the big question is whether or not the recalibration can be done "on the fly" (like with a knob) or requires more fiddling.
Fred's use of "yellow box" was a pretty good recommendation for me.
my is 5 % off which is a lot different then 5 MPH off. Which means in a 25 mph zone, if you go 5 mph over, you are speeding. Once you reach the speed of 60 to 70 mph then 5 mph over will work. So be careful and what and when you add 5 mph.
I took care of the speedo error by putting an oversize Metzler on the rear. Corrected it to dead on; when I had gone 100 miles, the odometer would read 98 miles. I did notice some handling differences and when I popped the rear tire with a nail, I went back to the standard size.