2001 740i (E38) balance issues or so I think

[BMWinMB]

I have a 2001 740i (E38) with two sets of rims. Currently running the following the following wheel/tire sizes, Front 245 / 40 / 20 and in the Rear 275 / 35 / 20, the other set are 19's. I get what seems to be a shaking sensation at various speeds depending on road surfaces and have had my wheels balance twice, thrust bushings replaced and the alignment checked twice. The problem has gotten worse with tire wear and my mechanic says the rims are simply two large and I should go back to 18's. The local tire store said there are no suggested listings for 20 inch rims for a 2001 E38, yet I seem to see other members running 20's without incident. I had the same problems with the 19 inch rims too, but not as bad.

I believe the rims are BMW OEM riims, but don't know the offset. Is there a way to tell my looking at the rim? How does one determine wheel offset? Tires are Continental Extreme Contacts.

Any ideas on what the problem is and the solution to fix it?

[BMWinMB]

Here is a possible solution, anyone has additional comments? http://www.meeknet.co.uk/e38/E38_Shimmy.htm

[Mless5]

Offset will be stamped, usually inside of the rim. ET or IS followed by double digits.
You could have simply bent wheels. Have you had a tire shop spin them on the balancing machine to see if there is a side to side or up and down oscillation?
Do you have any locals who could lend you a set of known true wheels for you to throw on your car for a day?
Post a picture of your wheels.

I believe the rims are BMW OEM riims, but don't know the offset. Is there a way to tell my looking at the rim? How does one determine wheel offset? Tires are Continental Extreme Contacts.

Any ideas on what the problem is and the solution to fix it?