The general formula formula for tire diameter given the www/hhRrr info is:
((2 x www x .hh) / 25.4) + rr
Where www is the tread width, hh is the height%, rr is the rim diameter, and 25.4 does the mm to inches conversion.
So 275/70R18 would work out to:
((2 x 275 x .70) / 25.4) + 18 = 33.16 inches.
This is the typical height as measured on the appropriate rim. For example most tire mfrs quote the diameter of the 265/75/R16 tire using a 7.5" rim and many users are putting the tire on 7" rims. This causes the tire to crown a bit and increases the tire diameter a bit.
Finally, the effective diameter of the tire rolling down the road is somewhat less than the value given above because the tire won't be perfectly round with a load on it.
The spec sheet from the mfr usually lists the turns per mile as well as the tire diameter. The turns per mile will give you the circumference to use in gearing and mileage calculations and you can get the effective rolling diameter from that.
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