I had the same problem when I got my new 1600x900 monitor, I set it by going into the racer.ini and typing in my resolution. Then you will need to set your render_aspect to 0 and racer will set the aspect ratio for you
I expect you can get it on tracciontrasera.es - it lets you set up lots of things, although being old, doesn't have some of the newer features, like motion blur, FBO, auto exposure, etcetera. But it's still very good.