34 inches beoch
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
My 99 F-250 SD came with LT235/75R16, but it will take up to LT275/75R16 on the stock 16" wheels.