305 is not a tire size. What is the complete size of the tire.
LT235/75 R15 (Canada)
17 inches
25.5 inches tall
35
Lt235/85/r16
32.72
17
23.44 inches
A tire that is 33 inches tall, 12.5 inches wide and is made for an 18 inch diameter rim.
215/70R20 is 32 inches tall But without a specific wheel width or desired tire width a "Correct" exact tire size is not possible. To figure out what you need search "Tire Size Calculator on Google and choose the Miata.net choice
The stock front tire size is 22x7x10. This means the tire is 22 inches tall, 7 inches wide, and fits on a 10 inch rim. The stock rear tire size is 22x10x9. 22" tall, 10" wide, 9" rim. The stock tire brand is Ohtsu.