What two cities are farthest away from each other on the earth's surface?

Taipei (Taiwan) - Asuncion (Paraguay) with a distance of 12,186 miles (19,916 KM). The circumference of the Earth is 24,900. SO, divide that by two and the max two points could be from each other is 12,450.

Singapore to Quito is 12251 miles.

The distance between Pekanbaru, capital of the Indonesian province of Riau, and Quito, Ecuador is somewhere between 12,406 Miles and 12,428 Miles. I say somewhere, because I have got those two different distances and don't known which is right. Either way it is certainly further than Taipei to Asuncion, and looking long enough at an atlas is enough for anyone to see the distance must be further than that of Singapore to Quito.

As an aside, the population of Pekanbaru is over 793,000 according to google Earth, and 903,902 according to wiki - not sure which to believe, the wiki entry seems a little too precise. Either way it fits the definition of a city by any relative standard, obscurity aside.

The greatest possible distance is, to be precise, 12,439 Miles (20,020Km), so the upper distance calculation I got for Quito to Pekanbaru, is only 11 miles short of such a distance.