Road noise goes up because most people go to a lower profile tire when buying larger wheels. The lower profile, ideally, should maintain the same rotational diameter, so your speedo is off. Plus, as enthusiasts, we're drawn to the greater handling benefits of low-profile tires.
The downside to a lower profile tire is that there is less rubber and air between the road the wheel (which transmits shocks to the rest of the car), so there will be greater noise and more bumps. If you want less noise, look for a tire with a history of low noise or just get a bigger profile.
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