DionR
Well-Known Member
FWIW, I wouldn't even bother with backspacing on a rim. What is important (or maybe just easier) is offset and section width. Half the section width of the tire, +/- the offset (depending on if the offset is positive or negative) will give you a better guess at where the inside of the tire sits. For example, a 245/45-17 tire on a +30mm offset rim will sit about 6" from the mounting surface (half 9.6" plus 1.18"). With backspacing, you need to know how thick the bead is (adds to the backspacing) and then figure out how much the tire bulges (section width minus rim width, divided in half). Bulge minus bead plus backspacing gives you a number, but it's more of a guess.
In any case, the section width is dependent on the rim it's mounted on (a 275 on an 8 rim will pinch the tire and give a smaller section width than when mounted on a 9 rim, for example). In the end, it's all just an approximation.
Only reason I bring it up is a 255/40-17 tire on a 17x9 rim with +24mm of offset and 245/45-17 tire on a 17x8 rim with +30mm of offset are less than an 1/8 of difference in where the inside of the tire sits, despite the difference of almost a 1/4 of backspacing. Not much, but it might make the difference.
In any case, the section width is dependent on the rim it's mounted on (a 275 on an 8 rim will pinch the tire and give a smaller section width than when mounted on a 9 rim, for example). In the end, it's all just an approximation.
Only reason I bring it up is a 255/40-17 tire on a 17x9 rim with +24mm of offset and 245/45-17 tire on a 17x8 rim with +30mm of offset are less than an 1/8 of difference in where the inside of the tire sits, despite the difference of almost a 1/4 of backspacing. Not much, but it might make the difference.
