What does an alto sax sound like if you use a tenor mouthpiece with it?

An alto saxophone with a tenor mouthpiece will not produce the same sound as a traditional alto saxophone. The resulting tone will most likely be stuffy, dark, and lacking in the high-end brilliance that is characteristic of the alto saxophone. Additionally, the intonation will likely be off due to the mouthpiece being designed for a larger instrument. Additionally, the tenor mouthpiece is designed for a different type of reed, so the timbre, intonation and overall tone of the instrument may be affected when used on an alto sax.