Do you even know which witness you are accused of tampering with? Are you sure it's the complainaint?
There are theft offenses which involve your obtaining property with the owner's consent - for example, larceny by false pretenses and larceny by conversion. You don't exactly paint a portrait of yourself as an innocent, hocking the bike the day after you borrowed it.
Your sentence will depend, among other things, on your record. You really need to consider getting a criminal defense lawyer.

