Hi,
Wheel/tire combinations can be a little dicey, especially when you're going fatter than stock.
This is the way I understand offset. The number on the wheel is the distance in millimeters that the mounting surface of the wheel is from the vertical centerline of the wheel. So, a 35mm offset from centerline actually positions the wheel closer to the fender lip than a 43mm offset. Since offset is independent of rim width (remember,it is measured from wheel centerline), an increase in rim width from 6.5 to 7 inches positions the outer rim .25 inches further from the hub (and closer to the fender lip), everything else being the same.
This is hard to visualize sometimes, so laying it out on a piece of graph paper can be helpful.
Also, a 205mm tire is always about 205mm wide, +/-. The aspect ratio, 60 and 55 in your case, means the sidewall height is 60% and 55% respectively of the tire width, 205mm. So the 55 series tire is "lower profile", and has a smaller circumference, which means it has to go around a few more times every mile, which makes your speedometer read fast.

Hope this helps,
Mark