G'day Kris,
Not sure about whether each printing place is different or not, but I've recently printed some of my holiday shots at various sizes up to 20x30 inches, and I gave the printers the max resolution tiff files I had...
I've even upsized one as an experiment to a 20M pixel image (single layer 45Mb tiff) and they simply print it at what-ever size I ask for, the printer opens them using photoshop... I haven't down-sized any images for printing, I leave them at max resolution...
Dunno if that helps... Other more experienced guys in this forum will probably give you better info.
The shot itself looks great (I'm a car person too, so nice

), the only thing I noticed was if you go around the car's perimeter, check for little things that might distract once printed... There's a couple of things you might prefer to clone out, like the top of a building that sticks out directly above the driver side headlight, about where the passenger external mirror would be, and directly above the driver side external mirror (dunno if it's the tip of an aerial or a tree)... and definately with the caption I reckon...
Nice
Cheers,
Mudder