I have a dog that looks very similar...we cannot figure out what she is either. We got her from the pound here in Pennsylvania and they had her listed as a terrier (pit bull I guess)/dalmation mix. Our vet suspects that she is a greyhound/ lab mix. That might make sense but she is only 40 lbs so those dogs would make her larger I would have thought. I have found info on plott hounds and it seems very siliar to her in personality and size but her ears and mouth are a little different. I don't know if we'll ever know what she is but we love her anyway
Here are some pictures of our girl: