You have to remove the speedo from the car and open it up on the bench. The "ribbon" is actually a red shape painted on a drum. A spinning magnet inside one end of the drum causes it to rotate while a coiled "watchspring" on the other end tries to return the drum to zero. So in your case, the spring needs to be tightened in the direction that will create more resistance to the drum being advanced.

You can probably bench-test the speedo with drill motor, both before and after the spring adjustment to see what effect it had. (Just make sure the drill motor is turning exactly the same speed both times.) That would tell you if you're getting close before you go to the trouble of reinstalling it.

25% is a lot to correct with a spring adjustment. You may be able to get it to read right-on at one speed, but not-so-right-on at another speed. But hey, at least you can get it closer.