Greyhounds do have a very high metabolism. All sight hounds do, as do cheetahs. The amount of food going in when balanced with excersise should mean that most pet Greyhounds would be obese. It is VERY unlikely that you see a fat Greyhound in the coventional sense like a Lab, for instance. The three rib rule, as above, is rule of thumb. You should just be able to see the third rib in your "average" Grey. A fat Greyhound tends to have a very wide back and a very boring life. A thin Greyhound is not a disaster, as my mother believes, it is often the case that a couple of pounds under on a 70lb dog looks dreadful. A Greyhound that looks right is just, well, perfect.
Dogncatluver has a beauty, Jamey, that has just joined her family and she has promised to post pictures as soon as she can - he could carry another pound or two but for a happy, healthy dog you couldn't ask for a better specimen!