Last week I had 3 emails about removing stickers and decals from Motor Cycles and the best tool to use.
To answer this question, you need to determine whether the sticker has a lacquer on top of the surface. You can usually tell if a lacquer has been used if the sticker has a smooth surface and you can not find an edge on the sticker.
If there is no lacquer, then the Wonder Wheel or Wonder Blade work very well in removing the decal.
The Wheel works by wearing away the decal and the blade by getting underneath the edge of the vinyl and sliding it off.
If the gas tank has a lacquer finish, then this surface layer needs to be removed and the only way we know is with low to medium grade sand paper. This can be a slow job, though the only option we know.