Before I get into it, I need to make a correction. It looks like my stock wheels are 7.5", not 8.5". Going from a 7.5" wheel to a 9.5" wheel, without increasing the tire width, means it's getting stretched 1" on each side.
I'll start from the beginning since I'm not sure where your knowledge of tires is. For a 19x9.5 wheel, 19" would be the diameter of the wheel, and 9.5" would be the width. For a 255/45R19 tire, 255 is the width of the tire in mm, 45 is the sidewall height as a percentage of the tire width, and 19" is the size of the wheel it's meant to mount to. As an example, my stock wheels and tires are 17x7.5 and 255/55R17. If you multiply 7.5" by 25.4 you get 190.5mm. Tire widths go up in increments of 10mm, so assuming you wanted
practical options, your sizes would be 195, 205, 215, or 225, which is what my car had from the factory. A wider tire provides better grip, so wider is generally better up to a point.
So, with a 9.5" wheel, that would be 241.3mm, so rounding that up to the next tire size would be 245, but of course, you could fit a 255 or maybe wider on there. I wasn't concerned about how much rubber I could fit on my wheels since I don't do any kind of racing or aggressive driving, so maybe you could go wider if that was your goal.
Sidewall height gets into kind of a weird area because what you pick depends on a lot of factors:
- Are you trying to match the factory tire diameter, keeping stock clearances and speedometer accuracy?
- Taller sidewalls are more comfortable and transfer less road noise but
- Shorter sidewalls look cooler
- Availability of tire sizes, especially given tire type and cost
I'll assume you want to keep the same outer diameter as your stock wheels, so in effect, you're only growing the size of the wheel inside the tire. The outer diameter of the tire is your sidewall height x2, plus your wheel diameter. Below is a chart where I've calculated the sizes of different tires so you can get an idea of what your options might be. Keep in mind that, while the math here is simple, the sizes are still approximations because of where the tires are measured, stretch, and other tire deformations are not taken into account. I'm also rounding to the nearest mm.
| 225/55R17 | 225/40R19 | 225/45R19 | 245/35R19 | 245/40R19 | 255/35R19 | 255/40R19 |
Wheel Diameter (mm) | 432 | 483 | 483 | 483 | 483 | 483 | 483 |
Sidewall Height (mm) | 115 | 90 | 101 | 86 | 98 | 89 | 102 |
Tire Diameter (mm) | 661 | 663 | 685 | 654 | 679 | 661 | 687 |
So as you can see, if you're trying to match the same outer diameter as the original tire and you want a "properly" sized tire, 255/35R19 would get you there exactly. There are plenty of other options that will get you close, depending on what look and performance you're aiming for. Once you've identified a tire size you think is good, search on TireRack to make sure it's a common size. If you're set on going for a stretched tire, just keep in mind that, because the sidewalls are now at an angle, the diameter of the tires is going to change, so you may want to go up 5% or something to compensate.
If you haven't already purchased your new wheels, you'll also need to take into account the wheel's offset and how much clearance you have between the inside of the wheel and the suspension, as well as the outside of the wheel and the fenders.
Edit: One consideration I forgot to mention is that, with stretched tires, you're also leaving your wheels more exposed and prone to curb rash. Not a huge deal but it's heartbreaking when it happens.
Edit 2: Not Mazda but very relevant