whats the best thing to use to remove tar on a carpet  

You could try using white spirits. Try a bit on the carpet somewhere hidden first to make sure it does not "eat" the colour or anything else.

I had the same problem earlier in the year following the Highways Dept "re-dressing" the roads outside. The stupid cat belonging to the ex walked across the street and straight through the lounge with tacky bitumin on it's feet, leaving nice little black slodges all through the cream lounge carpet. The f'ing dirty, stinking, flea-ridden little bar steward!!!

The company that did my fascias and soffits left me some of their cleaner, which is spirit based and worked a treat. Put plenty on a cloth and rub well in, left it for a few mins, then put a bit more on with a clean cloth and hey presto!! Jobs a guddun.

There's no tar/bitumin there any more, but the carpet has melted and disintegrated away. No, only joking. Given it was spirit based, I tested it first on some off cut of the carpet in case it leached the colour out and made it worse.

Moral of the story for me. SHould have bought a big angry dog to keep the cat out!!