Step Rings - vignetting

I have 58mm lenses and a number of 55mm filters, will I get vignetting if I use an adapter 58 - 55.

Probably not if you are using a crop camera. But it depends on the camera and lens. What camera and lens do you have?

Why not try it and tell us.

Thanks, I will.

I have an XTi with 18-55 lens.

If you use a regular EF lens there would be less chance of vignetting because only part of the image circle is used on a cropped sensor body.

The 18-55 is an EF-S lens and it was designed for the smaller sensor of a crop camera.
It may vignette, but the only way to know for sure is to try it.

A good rule of thumb when using stepping rings is "go up but not down." In other words, you can use a 58mm filter on a lens with a 55mm thread, but should not use a 55mm filter on a 58mm thread lens.