The 63 mpg figure is probably taken using imperial gallons. In US gallons it comes out to 52.5 mpg, still not bad. Imagine the results if "modern" technology was implemented in an older (and lighter) car, mpg's would be in the 70's....
Of course "modern" diesel technology means manufacturers can run much lower compression ratios than in the past, allowing them to use lighter (and less durable) materials. Once advanced technology on a diesel, aluminum cylinder heads are now the norm, some engines even have an aluminum block. Yet diesels continue to spin at higher speeds than ever before with increased emissions equipment, which could mean that "modern"* diesels may not be the durability kings that their ancestors were.
*"modern" because technology such as direct injection in diesels has been around since the 1920's....really modern stuff. A "proper" diesel would be an engine that needs no electricity or computer (other than the starter) to run, none have been installed in a car in the US since 1995.