I would suggest a go using a clay bar. I just went over my car as it had black spots all over. Tar remover didn't remove some of it but the clay bar did. Really impressed and car looks brand new now. A must for any white car owner I would say! The one I have got is:
http://www.camskill.co.uk/products.php?plid=m37b78s3519p817&rs=gbIt's not tar though, up close you can see the paint pitting in where it's dented the paint and chipped it off. Has a sandpaper like texture too. Not too fussed as i said before, it's basically underneath the car and can't be seen unless you lay down, but i can imagine how it would annoy a true perfectionist! Driving home just now in the MiTo with new tyres, sounds like you're driving in a hail storm the amount of chippings and stones on some parts of the road is bloody disgraceful!
Donnelly Bros, proud supporters of Abarthisti