Since this seems to coming up once per week, I thought I'd make a quick post about tire sizes.
Note: Some of these sizes won't actually exist while others will only exist for "truck" tires (which is fine).
This post assumes that your wheels are fairly close in dimensions (width, offset, backspacing, etc) to the OE wheels.
Ideally, you want to keep the new size within 3% of the original size. If you have AWD, you want all four tires to be the same size, make, and model (because tires of different make/model with identical "sizes" can actually be noticeably larger or smaller than each other).
This list is by no means comprehensive. With correct attention to detail and fine tuning, you can probably go wider than 245. However, as you go with wider tires, you'll need wider wheels. If you go with wider wheels, you'll need to start paying attention to wheel offset otherwise you may find that the wheel or tire interferes with suspension components (i.e. you'll need go with a lower offset to push the wheel out a bit). But, if you push the wheel out too far, the tires may rub on the fenders.
Tire sizes that should fit OEM wheels
16"
17"
18"
Tire sizes for non-OEM wheels.
17", wider wheel recommended
* You can probably get these to fit on the OEM-like 17x7 wheels but I don't recommend it. You'll want at least 17x7.5, preferrably 17x8.
18", wider wheel recommended
*** You can probably get these to fit on the OEM 18x7 wheels but I don't recommend it. You'll want at least 18x7.5, preferrably 18x8.
19"
20"
These data were produced using the Miata.net tire size calculator.
Note: Some of these sizes won't actually exist while others will only exist for "truck" tires (which is fine).
This post assumes that your wheels are fairly close in dimensions (width, offset, backspacing, etc) to the OE wheels.
Ideally, you want to keep the new size within 3% of the original size. If you have AWD, you want all four tires to be the same size, make, and model (because tires of different make/model with identical "sizes" can actually be noticeably larger or smaller than each other).
This list is by no means comprehensive. With correct attention to detail and fine tuning, you can probably go wider than 245. However, as you go with wider tires, you'll need wider wheels. If you go with wider wheels, you'll need to start paying attention to wheel offset otherwise you may find that the wheel or tire interferes with suspension components (i.e. you'll need go with a lower offset to push the wheel out a bit). But, if you push the wheel out too far, the tires may rub on the fenders.
Tire sizes that should fit OEM wheels
16"
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
205/60-16 4.8in 12.8in 25.7in 80.7in 785 -2.9%
205/65-16 5.2in 13.2in 26.5in 83.2in 761 0.1%
215/60-16 5.1in 13.1in 26.2in 82.2in 771 -1.2% (Factory Sport tire size)
215/65-16 5.5in 13.5in 27.0in 84.8in 747 2.0%
225/55-16 4.9in 12.9in 25.7in 80.9in 783 -2.7%
225/60-16 5.3in 13.3in 26.6in 83.7in 757 0.6%
235/55-16 5.1in 13.1in 26.2in 82.2in 770 -1.1%
235/60-16 5.6in 13.6in 27.1in 85.1in 744 2.4%
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
205/60-17 4.8in 13.3in 26.7in 83.8in 756 0.8%
215/55-17 4.7in 13.2in 26.3in 82.7in 767 -0.6% (Closest matching 17" tire size)
215/60-17 5.1in 13.6in 27.2in 85.3in 743 2.6%
225/50-17 4.4in 12.9in 25.9in 81.2in 780 -2.3%
225/55-17 4.9in 13.4in 26.7in 84.0in 754 1.1%
235/50-17 4.6in 13.1in 26.3in 82.5in 768 -0.8%
235/55-17 5.1in 13.6in 27.2in 85.4in 742 2.7%
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
205/55-18 4.4in 13.4in 26.9in 84.4in 750 1.6%
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0%
225/45-18 4.0in 13.0in 26.0in 81.6in 777 -1.9%
225/50-18 4.4in 13.4in 26.9in 84.4in 751 1.5%
235/45-18 4.2in 13.2in 26.3in 82.7in 766 -0.5%
235/50-18 4.6in 13.6in 27.3in 85.6in 740 3.0%
17", wider wheel recommended
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
245/50-17 4.8in 13.3in 26.6in 83.7in 757 0.7% *
245/45-17 4.3in 12.8in 25.7in 80.7in 785 -3.0% *
18", wider wheel recommended
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
245/40-18 3.9in 12.9in 25.7in 80.8in 784 -2.8% **
245/45-18 4.3in 13.3in 26.7in 83.8in 756 0.8% **
19"
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
205/45-19 3.6in 13.1in 26.3in 82.5in 768 -0.8%
205/50-19 4.0in 13.5in 27.1in 85.0in 745 2.3%
215/40-19 3.4in 12.9in 25.8in 81.0in 783 -2.6%
215/45-19 3.8in 13.3in 26.6in 83.6in 758 0.6%
225/40-19 3.5in 13.0in 26.1in 82.0in 773 -1.4%
225/45-19 4.0in 13.5in 27.0in 84.7in 748 1.9%
235/35-19 3.2in 12.7in 25.5in 80.0in 792 -3.7%
235/40-19 3.7in 13.2in 26.4in 82.9in 764 -0.2%
245/35-19 3.4in 12.9in 25.8in 80.9in 783 -2.7%
245/40-19 3.9in 13.4in 26.7in 83.9in 755 1.0%
Code:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/50-18 4.2in 13.2in 26.5in 83.1in 762 0.0% (Factory Touring and Grand Touring tire size)
205/40-20 3.2in 13.2in 26.5in 83.1in 762 -0.0%
205/35-20 2.8in 12.8in 25.6in 80.6in 786 -3.1%
205/40-20 3.2in 13.2in 26.5in 83.1in 762 -0.0%
205/45-20 3.6in 13.6in 27.3in 85.7in 740 3.0%
215/35-20 3.0in 13.0in 25.9in 81.4in 778 -2.0%
215/40-20 3.4in 13.4in 26.8in 84.1in 753 1.2%
225/35-20 3.1in 13.1in 26.2in 82.3in 770 -1.0%
225/40-20 3.5in 13.5in 27.1in 85.1in 745 2.4%
235/35-20 3.2in 13.2in 26.5in 83.2in 762 0.0%
245/30-20 2.9in 12.9in 25.8in 81.0in 782 -2.6%
245/35-20 3.4in 13.4in 26.8in 84.0in 754 1.1%