The only way to check the computer is by looking at the display. As long as the display is consistent, with no "jumping around" at high speeds - the pickup and MPH are most likely working correctly.

Another test is to pivk up the front of the bike and spin the wheel slowly. Again, you should note that the wheel displays all speeds as it comes to a stop. Usually, you can expect 3.0 2.5 2.0 1.5 etc......

If the unit quits measuring at 3mph or higher, remount the whee-magnet/sensor.......

It is true that old sending units wil fail and lose their ability to "send" very rapid signals - but when this happens, as I have noted, you should see an erratic MPH on the display -- like 35.5, then 20.0, then 34.5, then 22 etc.......