305 is the width in mm
50 is the % of that mm on each side (sidewall height)
So 50% of 305 mm is 152.5 mm on each side which is 6" each side (25.4mm=1")
so 6"x2=12" add in the rim height (20") an bam 32"
32.007874" to be exact.
Mack: 2014 GMC Sierra gas 5.3
Previous owner of many duramax's fast and slow.
Using Macky's formula and throwing in 55 for the sidewall ratio, you come up up with 33.20866. Using the website Mack posted, it calculates to 33.21. Are you measuring the tire loaded on your truck? Would it make a difference measuring a tire & rim combo loaded on the vehicle versus off the vehicle laying flat on the ground?
2008 Chevy LTZ CC/SB
EFI DSP5 SPADE
Profab EGR delete / Profab DP
Line X Bedliner