New axle ratio = stock axle ratio × (new tire diameter ÷ stock tire diameter). Starting from a common 3.73 stock axle on a ~31.5″ tire, that works out to roughly 3.91–4.10 for 33s, 4.10–4.30 for 35s, and 4.30–4.56 for 37s; round to the nearest ratio your axle offers.
Bolting on bigger tires is the most common first mod on an overland build (see the real cost of going from 33s to 35s for what that upgrade runs), and the one most likely to quietly wreck how the truck drives. A taller tire turns fewer times per mile, so at any given road speed the engine spins slower. That sounds efficient, but it isn't: the engine drops below the rpm where it makes useful torque, the transmission hunts for gears on grades, towing gets miserable, and real-world fuel economy usually gets worse, not better. Regearing, swapping the axle ring-and-pinion to a numerically higher ratio, puts the engine back where the factory intended.
The formula
The math is simple and exact. To restore your stock effective gearing after a tire change:
New axle ratio = stock axle ratio × (new tire diameter ÷ stock tire diameter)
Then round to the nearest ratio your axle is actually available in (common options: 3.73, 4.10, 4.56, 4.88, 5.13, 5.38). Diameter is what matters, not the marketing size. A metric “35” like the 315/70R17 is only about 34.4″. Use the real published or spec diameter when you can.
Recommended ratios by tire size
Most overlanding rigs leave the factory with a stock tire around 30.5–32″ in diameter. Starting from that baseline, here is roughly where you land for each popular tire size. Find your stock axle ratio on the door-jamb sticker or in the build sheet, then read across.
| Stock ratio | 33″ tire | 35″ tire | 37″ tire |
|---|---|---|---|
| 3.73 | 3.91–4.10 | 4.10–4.30 | 4.30–4.56 |
| 3.91 | 4.10 | 4.30–4.56 | 4.56–4.88 |
| 4.10 | 4.30–4.56 | 4.56 | 4.88–5.13 |
| 4.56 | 4.88 | 5.13 | 5.38 |
These assume a ~31.5″ stock tire. If your factory tire is shorter (a 3rd-gen Tacoma or a 2019–2023 Ranger ships on about 30.5″) the jump is bigger; if it is taller (some half-tons ship on 32–33″), the jump is smaller. Always confirm against the formula above with your real numbers.
By vehicle: common starting points
Stock sizes and ratios below come from the manufacturers' spec sheets; the ratios after them are the formula's answer rounded to the nearest ratio the aftermarket sells. Treat them as a sanity check on your own calculation, not gospel: trims and model years ship with different stock tires and ratios, and owners who tow or carry a heavy build often go one step higher than the formula.
- Jeep Wrangler JL / Gladiator (Rubicon ~33″ on 4.10; Sport/Sahara ~30–32″ on 3.45–3.73): from a Rubicon, 4.56 for 35s, 4.88–5.13 for 37s.
- Toyota Tacoma (3rd gen, 2016–2023) (265/70R16 or 265/65R17, 30.6″, on 3.91): the formula gives 4.22 for 33s, so 4.30; 4.47 for 35s, so 4.56; 37s want 4.88.
- Toyota Tacoma (4th gen, 2024+) (245/70R17 to 265/70R17 and 265/65R18, 30.5–31.6″, with TRD Pro and Trailhunter on 265/70R18 at about 33″; 3.58 on the automatic): from a 31.6″ tire, 33s compute to 3.74 and 35s to 3.97, so 3.73–3.91 and 3.91–4.10. The Pro and Trailhunter already carry a 33 on stock gearing.
- Toyota 4Runner (5th gen, 2010–2024) (265/70R17, 31.6″, on 3.73): 3.91 for 33s, 4.10 for 35s (4.13 by the formula; many owners go to 4.56 for a loaded rig).
- Toyota 4Runner (6th gen, 2025+) (245/70R18 on the SR5, 265/70R18 on TRD Off-Road, Pro and Trailhunter, 31.5–33″; 3.58): same arithmetic as the new Tacoma. From the SR5's tire, 33s compute to 3.75 and 35s to 3.98; from an Off-Road's 32.6″, 33s are no change and 35s compute to 3.85, so 3.91.
- Ford Ranger (2019–2023) (255/70R16 to 265/60R18, 30.1–30.6″, 3.73 on every trim): 33s compute to 4.04, so 4.10; 35s to 4.28, so 4.10 if you run light or 4.56 if you tow.
- Ford F-150 (275/65R18, 32.1″, or 275/60R20, 33″, on 3.31–3.73): 33s are barely a change (3.55 becomes 3.65, so 3.73); 35s from 3.55 compute to about 3.9, so 3.73–4.10.
Why the rpm penalty is the real story
The reason this matters isn't bragging rights: an under-geared engine spends its life off the torque curve. Go from a 31.5″ tire to a 35″ without regearing and the tire is 11% taller, so you lose 10% of your effective gear ratio. At 65 mph in top gear that pulls cruising rpm down by the same 10%, about 200 rpm on an engine that cruised at 2,000, enough to push the torque converter to unlock and re-lock on every grade and force more frequent downshifts. The calculator's rpm table shows this directly: it compares your current gears on the new tires against the recommended ratio, so you can see exactly how much rpm you're giving up before you spend a dollar.
Caveats worth knowing
- Regear both axles. On a 4WD vehicle the front and rear ratios must match, or you'll bind the drivetrain in 4WD.
- Recalibrate the speedometer. New gears (and new tires) throw off speedo, odometer, ABS, and traction control until the vehicle is reflashed.
- Mind your crawl ratio too. A numerically higher axle ratio also multiplies your low-range crawl ratio, a welcome side effect for technical trails.
- Diesels and high-torque engines tolerate taller gearing better than small gas engines, so they can sometimes skip a step.
Bottom line
Run your exact stock tire, new tire, and current ratio through the Gear Ratio Calculator. It returns the ideal ratio, the nearest available ratio, and the rpm difference at highway speed, including an optional overdrive ratio so the cruising-rpm numbers reflect how you actually drive. If the recommended ratio is within about 5% of stock, regearing is optional; beyond ~10%, it's the single best money you'll spend to make a big-tire build drive like a vehicle again.