This has probably been posted before but ... If you have a 97 Cobra and change out the rear gears to a 3:73 you will need to buy a new speedo gear. Part # 53682 at American muscle. 21 tooth RED gear.
A lot of people recommend the calibrator boxes but I switched out the gear to the red gear when 373s were installed and never had a problem with the speedo jumping or whatever. Plus the mph at cruise is about + or - 1 mph when at cruise speed.
Now with 410s , that would be a different story. FYI, I have had the 373 gears in the car for about 3 yrs now and drive it every day.