Question: I'm using/learning geodjango + postgis, and I'm trying to calculate distance between 2 cities in Poland. First using WGS84/4326 I define 2 points, and then after transforming this points into spherical mercator 900913 i get distance which is about 40km too long. Which map projection should I use then? Where can I find proper srid's for different countries?

sorry for the delay, 2 cities (lat; long): Slupsk(54.465249;17.026062) and Koszalin(54.193378;16.179428)

Answer: 
Use latitude and longitude and the [great circle distance](http://en.wikipedia.org/wiki/Great-circle_distance), not a map projection.
Based on the 2 points in your comments:

```
2 cities (lat; long): Slupsk(54.465249;17.026062) and Koszalin(54.193378;16.179428) 

```

I used two different approaches. The formula given in [The Aviation Formulary](http://williams.best.vwh.net/avform.htm#Example) I ended up with a distance of 63.116745 KM

I also used this tool [Calculate distance, bearing and more between Latitude/Longitude points](http://www.movable-type.co.uk/scripts/latlong.html). I ended up with an answer of 63.16 km.

Both methods use a "cue ball" earth, they do not use WGS84, or any projection.