The Mirage (in either form) is a rolling tin can with virtually no sound insulation. It's going to have road noise. I wouldn't be surprised to learn that the 15" tires (Yokohama) aren't as quiet as the 14" Dunlops. But I wouldn't let that sway my purchase decision either way. The majority of the noise you are experiencing is lkely due to the car's complete lack of sound deadening material.
If you want to talk about making the car more quiet, check out some of the sound deadening discussion on here. Adding the right sound insulation (Dynamat or similar product) to the car can make a significant difference. I'm actually surprised that Mitsubishi hasn't figured this out for the US market. Road noise is usually one of the whiners (car reviewers) first complaints. Spend another $50 to insulate the floor of cars headed to the US market!
Regardless of which wheel/tire option you decide on, realize that Mitsubishi didn't do us any favors either way. Both the 14" and 15" tires sizes on these cars are completely whacky. It's like the bean counters went into a room and said "What are the 2 most unusual tire sizes for 14" and 15" wheels in the US? Whatever they are, that's what we're using!"
So we're stuck with 165-65-14 (which no other car besides the Mirage uses) and 175-55-15 (these are used only on the front wheels of some late model Smart cars). Then there's the issue of the wheels only being 4.5" wide in both 14" and 15" diameters. Yikes! Most people wisely abandon these odd sizes when they get new tires. By doing so, it will usually open up a better selection of all sorts of tires...including ones that are quieter.
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View my fuel log 2015 Mirage ES 1.2 manual: 52.2 mpg (US) ... 22.2 km/L ... 4.5 L/100 km ... 62.6 mpg (Imp)