Typically for vehicles you want to go around 1/35 to 1/48, 1/35 being on the "too large" side and 1/48 being a touch to small.
However in the context of cars, I think 1/35 or bigger is better for Orks, otherwise the wheels will look too small and in general the vehicle will look too small for an Ork to fit in the driver's seat.
You can even go down to 1:25, which will give you nice sized wheels and Orky sized driver's seat, but the overall vehicle might be a bit too large, so you could chop the chassis to make it have a smaller footprint.
But it all depends on the specific vehicle I think. Vehicles that are large to begin with (trucks and whatnot) you can use a smaller scale like 1/48, smaller vehicles things like cars you probably want to go much larger scales. Here's someone who has pictures of vehicles of various scales next to 28mm humans (remembering Orks are going to be bulkier again).
http://chicagoskirmish.blogspot.com.au/2012/03/28mm-on-cheap-sci-fi-bikers-and-post.html