For what little it may be worth, we have always tipped our private guides is Europe at about 10% -- up to 15% if the tour was really great. Tipping, it is true, is an American custom that has entered Europe due to American presence there. For example, some American mass market cruise lines (i.e., Royal Caribbean) have higher fares for English guests, because they include onboard tips in the fare for these, as the English don't tip like Americans do.

And another factor for us actually is the content of the tour compared with the price. If we feel the price/content relationship of the tour indicates it was a "rip off", we don't tip as much as we would if we felt this relationship indicated the tour was a good buy.

All in all, we do tip somewhat less in Europe than we do in the US.

Thanks,
Richard