I have a 17-40L F4 as my walk around lens. I also use it at times when i do automotive shoots and never really had a problem with the aperture being 4.
For car shoots, i would lean more towards the wide angle lenses. Canon 10-22, Tamron 12-24, Tokina 11-16 etc etc.
2008 Lotus Exige S240 | 2007 BMW 335i