tl;dr: 2880 km for the Viking 2 / Curiosity distance is wrong, apparently based on very out of date position information for Viking 2. A distance of 3114 km (from landing site to landing site) is more accurate.
Hm, well. It looks as if the position of the Viking 2 lander may have at one time been thought to be 44° N 226° W. Those coordinates were mentioned in a 1995 article in the Journal of the British Interplanetary Society as well as a book from 1996. But the earliest archived version of a web page from the National Space Science Data Center, dating from late 1996 (page says it was updated in August; it was archived in December), gives 47.57 N, 225.74 W. By February 1998 this had been revised to 47.97° N, 225.74° W; these coordinates are still used on the present version of the page. This 1998 JPL page says the position is at latitude 47.67° N 134.48° E. (Note use of east vs. west longitude.) In December 2006 it was announced that the Viking landers and Spirit rover had been imaged by the Mars Reconnaissance Orbiter, but no revised coordinates were mentioned in the announcement. This book says 47.97° N, 225.74° W with uncertainty “5 m (from MRO imaging)”, which is a little odd since those coordinates seem to date from 1998 or earlier, more than 8 years before MRO imaged the lander.
Anyway, it does look as though the latitude of 44° N may have been the estimate at one time, but it’s been just under 48° N since at least the mid 1990s. If that’s so then the Mars Mileage Guide has Viking 2 coordinates that are about 20 years out of date.
Plugging in those old coordinates, along with the Curiosity landing site coordinates stated on the same page, at the Terrestrial Planet Mileage Calculator gives 2880.7 km distance between Viking 2 and Curiosity. But using 47.97° N, 225.74° W results in 3114.2 km. That’s larger than the 3060 km I got from Google Earth. That even though I used Curiosity’s current location in Google Earth, and the rover’s roved a few miles south (further from Viking 2) from the landing site. Maybe due to inaccuracies or different assumptions in the Google calculations.
For now I’ll use 3114 km for the total distance.
As of tonight I’m 0.4% of the way there.