Will the rims on a 2008 CLS 550 fit on a 2008 E350?

There's a website called wheel-size.com, go there, enter the make, model and year for each model and do a comparison. The site gives you all the info you need including wheel stud pattern and wheel centre bore.

CLS 550 has 245/40R18 tires on the front and 275/35R18 tires on the rear with 28 and 33 mm offset respectively. The E550 (couldn't find a listing for a 2008 E350) has 245/45R17 tires front and rear. There's two main problems you might encounter which are that A) the speedometer will be inaccurate due to a different diameter tire and B) the wider rear tire might catch on the shock absorber or the edge of the wheel arch. Neither of which are desirable.