What gear ratio for 37 inch tires?
Answer
4.64:1
Going from 285/70R17 to 37x12.50R17 on a 4.10:1 axle, the ratio that restores the original engine speed is 4.64:1. The nearest ratio sold as a ring-and-pinion set is 4.70:1.
The working
- Starting axle ratio
- 4.10:1
- Diameter change
- +4.29 in (+13.12%)
- Ratio restoring original rpm
- 4.64:1
- Nearest ratio commonly sold
- 4.70:1
- Effective final drive before re-gearing
- 3.62:1
A 37 turns a 4.10 truck into a 3.62 truck
That is the clearest way to see what has happened. Fit 37x12.50R17 in place of 285/70R17 and the axle hardware is untouched at 4.10:1, but the engine now behaves as though it were connected to a 3.62:1 — which is roughly what a differently specified version of the same vehicle would have left the factory with. The tire has quietly performed a gear change, in the direction of taller.
Restoring it needs 4.64, and 4.70 is what exists
The exact requirement is 4.64:1. There is no 4.64 ring-and-pinion set; the ladder runs 4.56, then 4.70. Choosing 4.70 lands slightly past the original engine speed rather than short of it, so the corrected truck turns marginally faster at a given road speed than it did on its original tires. Choosing 4.56 leaves a little of the change in place. Neither is wrong; they miss in opposite directions.
There is nothing mysterious in the multiplier. 285/70R17 turns 616.6 times to cover a mile and 37x12.50R17 turns 545.1, so the axle ratio has to grow in the same proportion those two counts differ: 4.10 multiplied by 616.6 and divided by 545.1 is 4.64. Every re-gearing figure on this site is that one line of arithmetic.
From other starting points
- From 3.73:1 the requirement is 4.22:1, with 4.27:1 available.
- From 4.56:1 the requirement is 5.16:1, with 5.13:1 available.
The last of those is the closest match anywhere in this family, which is a coincidence of where the manufactured ratios happen to fall rather than anything about 37-inch tires.
The size of the step
13.12% of diameter increase separates 285/70R17 from a 37 — a change of 4.29 in, with 2.15 in of it under the axle. That is nearly double the proportional change of a 35-inch swap from the same starting size, and it is why the gearing question is asked about 37s far more often than about anything smaller. Run your own axle and speed to see every gear rather than just top.