What gear ratio for 33 inch tires?
Answer
3.89:1
Going from 265/70R17 to 33x12.50R17 on a 3.73:1 axle, the ratio that restores the original engine speed is 3.89:1. The nearest ratio sold as a ring-and-pinion set is 3.91:1.
The working
- Starting axle ratio
- 3.73:1
- Diameter change
- +1.39 in (+4.41%)
- Ratio restoring original rpm
- 3.89:1
- Nearest ratio commonly sold
- 3.91:1
- Effective final drive before re-gearing
- 3.57:1
The arithmetic, and then the catalogue
There is a computed answer and a purchasable answer, and on a 33-inch swap they happen to sit almost on top of each other. Starting from 265/70R17 on a 3.73:1 axle, the ratio that returns the engine to its original speed at any given road speed is 3.89:1. The nearest ring-and-pinion set anyone actually sells is 3.91:1. The formula has no opinion about which of those you can buy; it just multiplies the old ratio by the change in revolutions per mile.
The 33 swap is unusually well served
Run the same calculation from other common starting ratios and the pattern holds:
- From 3.55:1 the arithmetic asks for 3.71:1, and 3.73:1 exists — two hundredths out again.
- From 3.21:1 it asks for 3.35:1, and 3.31:1 exists.
That is not a coincidence so much as a consequence of history: the available ratio ladder was laid out around the same tire-diameter steps people have been making for decades, so the 4.41% diameter increase of a 33 tends to land near a rung.
What happens if nothing changes
Leave the axle alone and the effective final drive falls from 3.73:1 to 3.57:1. That is the figure to think in, because it is what the engine experiences — a taller tire is a taller gear, and the axle hardware has nothing to do with it. Whether a drop of that size is worth a differential rebuild is a question about the vehicle, the load and the terrain rather than about geometry, and not one this page can settle.
Change any input
The gear calculator with these values loaded will take a different starting size, a different axle or a different cruising speed and recompute both the exact and the nearest-available answers. If your starting size is not 265/70R17, that changes the result more than anything else on the page.