The critical things I look for in a car mount is if it positions the phone display where I like it, usually somewhere in the center area of the dashboard slightly below eye level, that a mount doesn't require permanent damage to the dashboard (adhesives or screws), that it holds the phone level and doesn't move around despite bumps in the road, allows relatively easy attach/detach of the phone itself, and doesn't create new rattle sounds. You'd be surprised how many car mounts don't fit all these requirements, if you have the same as mine, my advice is to stay away from any that are below $10, I've tried too many cheap ones to save a few dollars and regretted it. Size-wise, there are many car mounts that can handle the 640XL, thanks in no small part to the popularity of the iPhone 6+, which has almost identical physical dimensions as the 640XL. I'm going to go out on a limb here and say that if the car mount says it works for an iPhone 6+, it'll work for the 640XL.
All that being said, I was happy with the build quality of the Mountek nGroove, which slots nicely into the CD slot of a car's CD player, the reverse clamp mechanism was excellent. However, my next car didn't come with a CD player, so I had to look for something else. Also, if the car's cd slot is positioned lower on the dashboard, it could be too low to view GPS navigation depending on your preferences. I have also tried the vent-type mount, the Kenu Airframe +. The clamp was a little tight for the 640XL + plastic case, but the deal breaker was that the mounting design to the vent itself simply didn't fit the Ford-style ac vents in my car, so I couldn't really use and evaluate it and had to return it. The mount I'm using now that I'm very happy with is the iOttie Easy One Touch XL, it grips the 640XL well, and works perfectly with my car's dashboard, since it has a flat platform area at the top of the dashboard where the suction cup grips tightly - I thought after a few weeks it would fall off, but after 3 months+ it has stayed on tightly. I will try to take a picture if you like, but those are a few I can recommend, depending on your preferences and car. Hope this helps.
WW