What is the sidewall height of 285/70R17?
Answer
7.85 in
285/70R17 has an aspect ratio of 70, meaning the sidewall is 70% of the 285 mm section width: 199.5 mm, or 7.85 in.
The working
- Sidewall height
- 199.5 mm / 7.85 in
- Aspect ratio
- 70%
- Section width
- 285 mm
- Overall diameter
- 32.71 in
Every error in this number doubles
A tire's overall diameter is the wheel plus two sidewalls. So whatever uncertainty attaches to the sidewall figure attaches twice over to the height: assume a millimetre too much and the diameter is two millimetres out. That is the reason this dimension is worth pinning down precisely rather than rounding, and the reason a size code with an unusual aspect ratio produces a diameter people consistently mis-estimate.
199.5 mm, which is very nearly 200
Seventy percent of a 285 mm section lands half a millimetre under a round two hundred. Nothing was designed to produce that; it falls out of two numbers chosen for other reasons. It does make the mental arithmetic unusually easy — call the sidewall 200 mm, double it to 400, add the 431.8 mm of wheel, and the answer is within a millimetre of correct.
The same figure arrived at backwards
A 33x12.50R17 has an 8.00 in sidewall, and it gets there by subtraction rather than multiplication: the 17-inch wheel is taken out of the stated 33.00 in diameter and what remains is halved. Two tires, 7.85 in and 8.00 in of sidewall, computed in opposite directions from codes that share no field.
The rim diameter is not part of the calculation
Nothing in the sidewall figure depends on the wheel. A 285/70R18 carries the same 7.85 in, because the sum uses only the section width and the aspect ratio. Change the last number of the code and the sidewall holds still while the overall diameter moves by exactly the change in rim size — which is why an apparently large jump in wheel diameter produces a predictable, one-for-one change in tire height.
Almost half the tire
7.85 in at the top and 7.85 in at the bottom of a 32.71 in tire means 48% of the overall height is sidewall. For a light-truck size that is generous, and it is the practical reason 285/70R17 is chosen over the shorter-sidewall options that reach the same height on a larger wheel — there is more casing to deflect before anything reaches the rim.
Draw it against a lower-profile size of similar height to see where the difference sits, or read aspect ratio for what the middle number is a percentage of.