Regear Calculator
Changed your tire size and want your truck or Jeep to drive like it did before? Enter your current axle ratio and your old and new tire diameters. This tool gives you the exact ring-and-pinion ratio to regear to, plus the nearest commonly available gears - and shows the penalty if you keep your current gears.
⚙️ Your Setup
🛞 Tire Diameter Units
Old / Original Tire
New Tire
📋 Results
🧭 What This Means
Enter your specs above to see a recommendation.
📋 Commonly Available Axle Ratios
| Ratio | Character | Typical Use |
|---|---|---|
| 3.08 - 3.23 | Tall | Highway cruising, best MPG, stock-size tires |
| 3.42 - 3.55 | Moderate | Daily driver, light towing |
| 3.73 - 3.91 | Balanced | All-around, regular towing, 31-33" tires |
| 4.10 - 4.30 | Short | Off-road, heavy towing, 33-35" tires |
| 4.56 - 4.88 | Very Short | 35-37" tires, rock crawling |
| 5.13 - 5.71 | Ultra Short | 38"+ tires, dedicated trail rigs |
How the Regear Calculator Works
When you bolt on taller tires, you change the last "gear" in your drivetrain without touching a single component inside the transmission or differential. A tire is effectively a lever: its rolling circumference decides how far the vehicle travels per wheel revolution. Increase the diameter and each revolution covers more ground, so the engine turns fewer times for a given road speed. The result is a numerically taller effective final drive, even though the ring-and-pinion ratio stamped on your axle has not changed.
This calculator restores the balance. It finds the new ring-and-pinion ratio that gives you the same effective final drive you had before the tire swap - the gearing the factory calibrated your shift points, torque converter, and fuel maps around. It then snaps that exact number to the nearest ratios that gear manufacturers actually produce, because you cannot order a 4.21 ring-and-pinion off the shelf.
The core idea is proportionality. Effective final drive can be written as axle ratio multiplied by a reference tire diameter divided by the actual tire diameter. To hold effective drive constant when the tire grows, the axle ratio must grow by exactly the same proportion the tire grew. That single relationship is all the math you need, and it is why a 13 percent larger tire calls for a roughly 13 percent numerically higher gear.
The Math Behind It
The recommended axle ratio is:
New Ratio = Current Ratio × (New Tire Diameter ÷ Old Tire Diameter)
Worked example: a truck with 3.73 gears on 31-inch tires upgrading to 35-inch tires.
New Ratio = 3.73 × (35 ÷ 31) = 3.73 × 1.129 = 4.21
Since 4.21 is not a stocked ratio, the closest choices are 4.30 (slightly shorter, a touch more aggressive) and 4.10 (slightly taller, a touch better for highway MPG). Both land within a few percent of ideal.
To size a tire from its metric designation, the calculator uses the standard section-height formula:
Diameter (in) = Rim (in) + 2 × (Width_mm × Aspect ÷ 100) ÷ 25.4
For a 285/70R17: 17 + 2 × (285 × 0.70) ÷ 25.4 = 17 + 15.71 = 32.71 in.
The calculator also shows the penalty for not regearing - the change in effective final drive if you keep your current gears:
Effective Change % = (Old Diameter ÷ New Diameter - 1) × 100
Going from 31 to 35 inches yields (31 ÷ 35 - 1) × 100 = -11.4%. Your effective gearing drops about 11 percent, which is why the engine feels lazy and downshifts more after big tires.
Industry Standards & References
SAE J1100 (Motor Vehicle Dimensions) defines how tire diameter and rolling radius are measured, and notes that the loaded rolling radius is typically 3 to 4 percent smaller than half the free-standing diameter. This calculator uses free-standing overall diameter because it is the spec tire makers publish and the value gear vendors reference in their fitment charts. The Tire and Rim Association (TRA) and its European counterpart ETRTO standardize the metric size-to-dimension formula used here. Ring-and-pinion sets are manufactured to AGMA quality classes; aftermarket sets for common axles (Dana 44, GM 10.5, Ford 8.8, Toyota 8-inch) are usually AGMA Class 10 to 11.
Step-by-Step Example
Scenario: A Jeep Wrangler JL left the factory with 3.45 gears and 32-inch tires. The owner installs a lift and 37-inch tires and complains the Jeep "won't get out of its own way" and the 8-speed automatic constantly hunts between gears on grades.
Step 1 - Recommended ratio:
3.45 × (37 ÷ 32) = 3.45 × 1.156 = 3.99
Step 2 - Snap to available gears: 3.99 sits between 3.91 and 4.10. For a heavy, tall-geared automatic on 37s, most builders choose 4.10 (or even 4.56 for serious wheeling) to keep the engine in its torque band on the trail. The Jeep community frequently regears 37-inch JLs to 4.56 or 4.88 for exactly this reason.
Step 3 - Confirm the penalty of doing nothing:
(32 ÷ 37 - 1) × 100 = -13.5%
Keeping the 3.45 gears leaves the drivetrain about 13.5 percent taller than stock, which explains the sluggish response and gear hunting. Regearing to 4.10 restores roughly stock feel; 4.56 makes it noticeably more energetic than stock, at a small highway RPM cost.
Common Mistakes to Avoid
- Using tire width instead of overall diameter. The formula needs the full outer diameter. A "35x12.50R17" is 35 inches tall; the 12.50 is width. For metric sizes like 315/70R17, use Tire Size mode so the diameter is computed correctly (about 34.4 inches, not 315 of anything).
- Chasing the exact decimal. You cannot buy a 4.21 gear set. Rounding to the nearest stocked ratio is expected and correct. Obsessing over 0.1 of a ratio point is wasted effort - tire growth, loaded radius, and driving style swamp that difference.
- Ignoring driving intent. The calculator restores stock feel. If you tow heavy or wheel steep trails, deliberately pick the next ratio shorter (numerically higher). If you mostly cruise the highway, lean one ratio taller to protect MPG.
- Forgetting the speedometer. Regearing does not fix speedometer error by itself. You still need an electronic recalibration that reflects both the new tire size and the new axle ratio, or the gauge will read wrong and shift logic can suffer.
- Regearing only one axle on a 4WD. Front and rear ring-and-pinion ratios must match exactly. Mismatched ratios bind the drivetrain in four-wheel drive and can destroy a transfer case.
When to Use This Calculator
- Planning a tire upgrade. Before you buy 35s or 37s, see exactly which gear set restores your drivability so you can budget the regear into the build.
- Choosing between two gear options. When the exact number lands between two stocked ratios, compare both and decide based on whether you value highway MPG or low-end grunt.
- Downsizing tires. Swapping to smaller tires (for example, tall highway tires back to stock) makes gearing shorter; the tool shows the lower ratio that would restore stock feel and warns you the engine may now over-rev at cruise.
- Diagnosing a sluggish rig. If a customer added big tires and never regeared, plug in the numbers to quantify exactly how much effective gearing they lost and justify the regear.
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❓ Frequently Asked Questions
How do I calculate what gears I need for bigger tires?
Multiply your current axle ratio by the ratio of new tire diameter to old tire diameter:
New Ratio = Current Ratio × (New Tire Diameter ÷ Old Tire Diameter)
For example, 3.73 gears on 31-inch tires going to 35-inch tires need 3.73 × (35 ÷ 31) = 4.21, so 4.10 or 4.30 gears are the closest available options. This calculator does the math and snaps to real ratios automatically.
Do I have to match the exact ratio the calculator gives me?
No. Ring-and-pinion sets are only made in fixed ratios like 3.73, 4.10, 4.56, and 4.88, so you pick the closest available. A ratio numerically lower than the target gives slightly taller gearing (better highway MPG and lower cruise RPM). A ratio numerically higher gives shorter gearing (better acceleration and towing). Choose based on how you use the vehicle.
What happens if I run bigger tires without regearing?
Larger tires make your effective final drive numerically taller, so the engine spins slower for any given speed. That dulls acceleration, forces more downshifts, makes an automatic hunt between gears, raises transmission temperatures when towing or climbing, and often hurts real-world fuel economy because the engine lugs out of its efficient range. The bigger the tire jump, the worse the effect - use the "effective ratio change" output above to quantify it.
Does regearing fix my speedometer after a tire change?
Not by itself. Regearing restores drivability, but your speedometer reads off the transmission or transfer-case output, so both the tire change and the gear change affect calibration. Most modern vehicles need an electronic recalibration (a handheld tuner) that accounts for the final tire diameter and the new axle ratio. Older cable-driven speedometers need a different driven gear in the tailshaft.
How do I find my current axle ratio?
Several ways: check the RPO/build sticker in the glove box or door jamb, look for a metal tag on a differential cover bolt, decode the axle code in your VIN or build sheet, or count ring gear teeth divided by pinion teeth (for example 41 ÷ 11 = 3.73). You can also jack up one wheel, turn it exactly one full rotation, and count how many times the driveshaft turns.
Is the diameter the same as the number in a size like 35x12.50R17?
For flotation sizes like 35x12.50R17, yes - the first number (35) is the nominal overall diameter in inches, so you can enter it directly in Diameter mode. For metric sizes like 285/70R17 you must calculate it: diameter = rim + 2 × (width_mm × aspect ÷ 100) ÷ 25.4. Use Tire Size mode and this calculator does that conversion for you.