Q: Can I replace my 225/45R18 tires with 215/55R17 tires? The 215's are what ALL tire sites say are original factory size

I owned a 2008 Sonata and currently own a 2015 Sonata Sport 2.0T Limited. With your generation as well as my 2015, the Sonata with the 2.0T engine has larger brakes so the 18 inch wheels make more room for them. Since yours is a 2.4 Limited, it only has the 18 inch wheels for appearance as all 2.4 ltr versions whether Base or Limited, have the same size brakes. I do not recommend a 215mm section width with the 18 inch wheels. The ride and handling will deteriorate badly. If you are sticking with the 18 inch wheels, then stick with the 225/45R18 tires. You can most certainly change down to the 215/55R17 tires AND wheels. Any 17 inch wheels listed for the Base, SE, GLS, etc will fit fine. The ride and handling will be different. It will ride a bit smoother as you are increasing the sidewall aspect ratio - 55 vs 45%. The will also be narrower - 215mm vs 225 mm. It may not grip as well in dry weather but you may notice a little better traction in wet and snowy conditions as there will be more weight concentrated on a narrower tread area. Your speedometer may be slightly off however. The average overall diameter of a 225/45R18 tire is 25.9 inches. So that makes for roughly 802 revolutions per mile. The 215/55R17 will have an average diameter of 26.3 inches and approximately 790 revolutions per mile. A slight difference that may only cause your speedometer to be off by 1-2 mph. On the up side, you may get a slight bump in miles per gallon. The taller tire will reduce engine rpms at highway speeds slightly. So if you are willing to reduce the wheels from 18 inch to 17 inch, then go ahead with the 215/55R17 tires. If you are keeping the 18 inch wheels, then stick with the 225/45R18 tires.