Does a set of rims that are 15 by 9 inch rims with a -20 offset seem too deep for my Lexus ls400?

 

Welcome to CL.

A first gen owner can chime about specific fitment. I suspect that if you are willing to do some fender mods, no problem. That depends what tire and if you are stretching it too. Are you opposed to going to a + size on your wheel-tire fitment?

I did a quick look around and this fitment guide for Rays came up. Should give you some ideas:
http://www.upgrademotoring.com/wheel...ment/lexus.htm

Rominl's wheel guide for some other models, a good primer BTW:
http://www.lgwnorcal.com/articles/wheelfitment.html