Grandpa must be driving a train, a steamship, or a strip mining shovel. As for our more typical mileage, well, half a million is kind of a big number, a little unwieldly for everyday conversation.
Wikipedia's explanation is that some old Viking branded (like with a red hot iron) his barrels of ale with the image of the head of an ox, so barrels of ale came to be known as ox-heads. By the time they got to North America a hundred years later, the term had been corrupted to hogshead. Or maybe somebody else started branding their barrels with the image of the head of a hog.
Just what do they call 'mileage' in countries that haven't been to the moon?
Update April 2016 replaced missing spreadsheet on account of Google changed the way spreadsheets are embedded.