Do the numbers change much going to 285/70R17?
Answer
3.86:1
Going from 265/70R17 to 285/70R17 on a 3.73:1 axle, the ratio that restores the original engine speed is 3.86:1. The nearest ratio sold as a ring-and-pinion set is 3.91:1.
The working
- Starting axle ratio
- 3.73:1
- Diameter change
- +1.10 in (+3.49%)
- Ratio restoring original rpm
- 3.86:1
- Nearest ratio commonly sold
- 3.91:1
- Effective final drive before re-gearing
- 3.60:1
The change is smaller than the gap between available ratios
This is the unusual case where the arithmetic produces an answer the catalogue cannot supply for a reason other than the number being extreme. Put 285/70R17 where a 265/70R17 was, leave a 3.73:1 axle in place, and the ratio that would put engine speed back where it was comes out at 3.86:1. The ladder of manufactured ratios steps from 3.73 straight to 3.91, so there is nothing at 3.86 and nothing close to it. Fitting 3.91:1 would move past the original figure rather than back to it.
What the effective ratio actually becomes
3.60:1. That is what the drivetrain behaves as though it has once the taller tire is on, and the entire change is 3.73 down to 3.60 — smaller than a single step on the ratio ladder, which runs 3.55, 3.73, 3.91. In other words this tire swap moves the truck less than one commercially available gear set would.
From other starting ratios
- 3.21:1 asks for 3.32:1, and 3.31:1 exists — an almost exact match, and the only one in this family.
- 4.10:1 asks for 4.24:1, and 4.27:1 exists.
- 3.55:1 asks for 3.67:1, with 3.73:1 the nearest.
Why the answer is so undramatic
The diameter change is 1.10 in on a tire that was already 31.61 in — 3.49%. Ratio requirements scale with that percentage, and 3.49% of 3.73 is about a tenth of a ratio point. A 33-inch or 35-inch swap moves the requirement by several times as much, which is why those questions have real answers and this one mostly does not.
What the gearing arithmetic ignores
Worth stating, because it is the part these calculations are silent about: the swap also adds 0.79 in of section width. Nothing in a ratio calculation sees that, since only diameter enters the formula. The gearing consequences of this change are minor and the clearance consequences may not be, and they are entirely separate questions answered by entirely separate measurements.
Set your own axle and speed and the calculator will show every gear before and after, or read axle ratio basics for what the ratio is measuring.