Hello and welcome.
What may seem like a simple thing to do will prove to be more dificult.
You are basically trying to make large car feel in a small car.
Mazda is all about the Zoom Zoom, which may come at a price of "American Car Feel"
It is unfortunate that you did not test drive the car a bit more.
As far as changing out the shocks. It will be very dificult to find a softer than OEM suspension set up.
Most are geared for Stiffer and Lower.. Not Softer and Taller...
Changing out the tires for a 15" Rim with a Big Fat Tire will have very horrible effects on the way the car turns and holds a corner.
I believe that anything you do to soften the car up, may also make it more dangerous to drive.
Have you thought about changing the seat, or getting some sort of seat cover that could reduce the pain in your back.
Thank you for input. You are perfectly right. I wish I had test drove the car a little bit more. On my "defense" I did a lot of reasearch online about the car and Consumers Report mentioned that the ride is sporty but not uncomfortable. The streets near the dealership were I test drove the car were in good condition, so I didn't feel the rough ride before I went on the highway.
I think I mentioned that I traded a Scion tc for this Mazda3. The Scion has more Zoom Zoom than the Mazda( more horsepower), is more fun to drive and a lot more comfortable. The one and only reason I traded the Scion was the need for a 4 door car. It became a little nuisance to help my daughter to get on the back seat.
As for the cushion, I'm already using the best one I found on the market(1" thick of foam), and it does make a little difference. I can't even imagine driving the car without it!
Yesterday I replaced my 205/55/R16 Bridgestone Turanza stock tires with 205/60/R16 Hankook Optimo H727, which were highly recomended for a comfort ride. I had hoped it will make a significant diffrence going for a taller, softer, tire. Unfortunately, $500 later there is no improvement. At all.
I also realized you are right about the shocks and struts. Thou the manufacturers(Monroe for example) advertise their products as meeting the OEM standards, but eliminating the stiff ride, I had a hard time believing.
So in conclusion I'm going to try to make peace with my mind and keep the car for as long as my back can hold
