I have a 2000 E430 with stock 17X7.5 in rims. I am running 245/45/17/tires and I am thinking of running 255/40/17s I do not drive the car hard, but want a little larger footprint. I like the rims I have and will keep them. I have pushed tire size before on other cars without any problems. I know what the "book" says, but we all know about "CYA" Anyone have any real experience or knowlege with this combo?
Thanks.

Hello JimE430, I hear what you are saying. If you want the tire store answer, it will be, "no". I too have "pushed the limit" with tire sizes on other cars too. Currently, I have 17X8's in the front with 245/45's and 17X9.5's in the rear with 275/40's. When I got my wheels a few years ago, the recommended rear tire size was 255/40/17. On a 9.5" rim, the rear tire was stretched out so much that the overall diameter was actually smaller than the front tires.

If you go with the 255/40/17's on a 7.5" rim, the sidewall will look bulged. Since you are considering this fitment on a stock rim, the only problem I could see would be the front fitment. The upper ball joint sits very close to the inside of the rim/tire. Most aftermarket rims in the 7.5 to 8.5 widths for the W210 use a 35mm offset, the stock is 41mm. The bulge of the 255 on the 7.5 wide rim may touch the upper ball joint. Turn your wheel to full lock on your car either to the left or right and you can see the upper ball joint.

The rear fitment will be fine. There is over 3/4" of clearance in the inside rear.

Typically, when putting a wider tire on a narrow rim, the feel is a little squishy. If you have invested in handling devices like stiffer shocks, lowering springs, and bigger bars, then doing this may be counter productiive even though you have more rubber on the road.