just a quick question, if i put summer mags on the cruze diesel and the wheel is 17x7.5 and offset is 42 mm is that ok with the goodyear oem tire 215-55-17 ?
Those wheels should work fine... as long as you get four. The pic shows a remaining stock of less than 3.
I did basically the same thing with my Eco; I'm using the OE forged Alcoa wheels with Michelin Xi3 winter tires in the factory size (same 215/55-17 as the diesel), and I bought a set of 17" alloys for summer use. My summer wheels are 17x8, a full inch wider than the OE wheel which is only 7", and the 215/55-17 GoodYear FuelMax tires fit fine. They are "stretched" a little more compared to a 7" wheel, but that actually looks a little better in my opinion.
As far as fuel economy goes, typically a smaller wheel will give better mileage (for a given tire OD) for several reasons. First, the wheel is smaller in diameter, so its rotational inertia (polar moment) is less for a given wheel weight making it consume/store less energy. Second, taller sidewalls generally offer lower rolling resistance (all else kept equal). We must keep things in perspective, though, and realize these are minute differences. For example, if a steel 16" wheel weighs slightly more than an aluminum 17" wheel, teh weight difference may make up for the rotational inertia advantage. It will vary case-by-case, and the differences will be slight in the grand scheme of things.
Generally speaking, winter tires offer much poorer rolling resistance than all-season tires do. Much of the mileage decrease people see when switching to winter tires has to do with the tire's construction, as well as decreasing temperatures.