The rims should be either 15's or 16's. The bigger rims themselves wont throw off your speedometer, just your acceleration times from the added weight. The size of the tired is what would through off your speedo. You can get tires that have the same circumference as your stock ones but will have a shorter side wall to accommodate the bigger rim. Go on discount tire.com and plug in your vehicle year make model(yadda yadda) it will then pull up your stock tire size and you can search for tires in that size but below that there is a link that says "view optional/plus sizes". Click that and it will give you a list off different size tires for different wheel sizes while keeping the overall circumference of the tires as close to the stock circumference as possible. Even if you're not gonna buy tires from discount(which I recommend cuz they have excellent service and have a large selection of tires) you can still go on the website and see what tires you can get without throwing off your speedometer.