Do i adjust the speedo or anything?

So i'd be interested in putting some snow tires on my 2.5i's stock 17in wheels and getting some other wheels. Question is, if i stay with stock subaru sizing, ie 215-45-17 from LGT or 215-45-18 from 3.0R, do all these cars have different speedo settings or does subaru just say oh well? Since the new wheels would be primary i want to try and get the milage almost correct.

Any advice?

Thanks

you want to stay with the same size tire/wheel circumference overall. it's called plus or minus sizing, as the rim size goes up the side wall needs to come down and vice-versa.

http://www.1010tires.com/tiresizecalculator.asp