Here's how I look at it:But aren't all Mazda's like that? I mean I thought SKYACTIV was basically a more lightweight material and the removal of a lot of things that do not affect safety but can otherwise be considered luxury (sound dampening/absorption material, more premium feeling material, etc.) to decrease weight and improve fuel economy.
Cabin noise has two major factors.
1) Noise generation.
2) Noise isolation.
Mazda limited the amount of noise isolation to reduce weight and increase road feel. We can't really change this part very easily. But, we can work to reduce the amount of noise generated. There are three main sources of noise generation: Wind, tires, engine.
It's easy to make wind and engine noises louder (spoilers, ground effects, intakes, exhausts, etc) but reducing it is tough. Honestly, I don't mind engine noise (though the idea of fake engine noise generated by the stereo seems kind of stupid to me). Wind noise is wind noise.
However, tire noise... We can change that and tires can make a HUGE difference. I replaced the OEM Bridgestone Potenza G019 Grid tires on my Infiniti because they were ludicrously loud (still had well over half their tread life left). The Bridgestone Potenza S-04 Pole Position tires that replaced them were amazingly quiet by comparison. It's even more surprising when you find out that I went from a 460 TW "Performance All-Season" to a 280 TW "Max Performance Summer" tire (rather than the other way around).
You can look at it the other way. Back when I was racing an NA Miata, I started off with Bridgestone RE-11A tires for racing. I couldn't hear the tread noise over the wind and other cars. Annoyingly, Bridgestone ditched that tire in my size and I had to switch. The newest hot tire for lighter cars was the Kumho Ecsta V720. It was a huge improvement in grip but holy God was the tread noise bad. I went from having the radio volume at 50% to 90% just to hear spoken words. All that changed was the tires.
Tires have a huge impact on various aspects of the driving experience. Tires spec'd by manufacturers for non-performance models are a compromise that focuses on whatever criteria they thought was important (often rolling resistance and how good a price they could get in volume purchases). They're also typically hugely marked up to the consumer when replacing them. It's usually pretty easy to find an aftermarket tire that can beat the OEM tire in the majority of criteria for less money.
Of course, you have to take a leap of faith: there's no good way to tell how a tire is going to sound on a car. Plunking down $500-$600 on a set of new tires (including shipping, mount, balance, disposal) is a tall order for most. This is why I'm going to try out the non-race wheels/tires from my RX-8 to see if my theory holds up. I'm going to try getting some dB readings with my phone before and after.
The stock size of 215/50-18 is an oddball but most tires in 225/50-18 (1.5% larger diameter) or 235/45-18 (0.5% smaller diameter) will work just fine. Most speedometers are 1-3% off anyway.