2009 BMW X5 xDrive35d

I've never had the opportunity to drive an X5 xDrive30i (that's German marketing-speak for the gasoline six-cylinder version), but I'd say upgrading to this diesel is a no-brainer. For about $4000 more -- a surcharge you'll likely get back in tax rebates and fuel savings -- you have a markedly more powerful, more efficient vehicle. Like all the modern diesels we've sampled from German automakers of late, this 3.0-liter has no obvious drawbacks compared with a typical gasoline engine. There's no hesitation, no smell, no cement-truck noises. All you notice is a slightly different mechanical sound.

David Zenlea, Assistant Editor