Find the ideal axle gear ratio for your cruise target, then compare it against real buyable gear sets.
Your setup
Cruise target
The ratio is set by the speed–RPM combination, so the same math works whether you’re chasing a speed target or an RPM target.
Share & saved setups
Worked examples
Ideal = 2000 × 28 ÷ (70 × 336.13) = 56,000 ÷ 23,529.1 = 2.38. Closest buyable set: 2.73 (+0.35), which runs ≈ 2,294 RPM at 70 mph (+294 vs target, −689 vs the current 3.55 set’s 2,983 RPM).
Ideal = 2200 × 33 ÷ (75 × 336.13) = 72,600 ÷ 25,209.75 = 2.88. Closest buyable: 2.73 (−0.15), ≈ 2,086 RPM at 75 mph (−114 vs target, −367 vs the current 3.21’s 2,452 RPM).
FAQs
What does an axle ratio like 4.10 actually mean?
Will taller gears hurt my towing?
Why don’t buyable sets match the ideal exactly?
Do bigger tires really need lower (numerically higher) gears?
Does this account for my transmission overdrive?
Where does the 336.13 constant come from?
Limitations
An axle ratio calculator tells you what ring-and-pinion ratio you need to hit a target RPM at a target speed — or, just as usefully, what your effective ratio becomes after a tire size change. If bigger tires have turned your truck into a dog off the line, this is the tool that tells you which gears fix it.
How to Use the Axle Ratio Calculator
- Enter your current setup: tire diameter (inches or mm), your current axle ratio, the speed you cruise at, and the RPM you’d like to see at that speed.
- Solve for axle ratio. Enter the RPM you want to cruise at and the speed you want to cruise at — the calculator returns the ratio that gets you there.
- Compare gear sets. The ranking table orders real buyable ratios (3.73, 4.10, 4.56…) by closeness to your ideal — pick the nearest one and check the ΔRPM column to see where it lands at cruise speed.
The Formulas
Axle ratio = RPM × tire diameter ÷ (mph × 336.13)
(This assumes a 1:1 transmission gear. In overdrive, divide the result by the overdrive ratio — e.g. divide by 0.70 for a typical overdrive — to get the axle ratio you actually need.)
Worked example: solving for axle ratio
You want to cruise at 70 mph at 2800 RPM on 30-inch tires, in a 1:1 gear:
- 2800 × 30 = 84,000
- 70 × 336.13 = 23,529.1
- 84,000 ÷ 23,529.1 = 3.5698 → ~3.57:1
No manufacturer sells a 3.57, so you’d pick the nearest available ratio — 3.55 or 3.73 depending on whether you want slightly lower or slightly higher cruise RPM.
For tire changes, the shortcut formula gives you the ratio needed to keep the same cruise RPM:
New ratio = old ratio × new tire diameter ÷ old tire diameter
Worked example: re-gearing after bigger tires
Going from 30-inch to 33-inch tires on 3.42 gears:
- 3.42 × 33 = 112.86
- 112.86 ÷ 30 = 3.762 → ~3.76
The taller tires cost you mechanical advantage — the engine turns slower for the same road speed, so acceleration feels lazier. To get the original feel back on 33s, you’d re-gear to roughly 3.73, the nearest ratio anyone actually sells.
Common Mistakes
- Forgetting the transmission gear multiplies the axle ratio. In a 0.70 overdrive, 4.10 gears act like 2.87 at cruise. Always include the transmission gear — otherwise your “needed ratio” will be way off.
- Using the nominal tire size instead of measured diameter. A “35-inch” tire often measures 34 or 34.5 inches mounted and loaded. Measure your actual tire, or your calculated ratio will be optimistic.
- Ignoring that front and rear ratios must match on 4x4s. Mismatched front/rear ratios bind the transfer case and destroy drivetrain parts. Re-gear both axles to the same ratio.
- Rounding to a ratio nobody sells. Ratios come in standard steps (3.42, 3.55, 3.73, 3.92, 4.10, 4.30, 4.56, 4.88, 5.13…). Pick the nearest real ratio, then check where it puts your cruise RPM.
- Chasing the exact stock RPM after a big tire change. Matching the old cruise RPM exactly can leave you lugging a heavier tire. Most builders aim slightly numerically higher than stock-effective for drivability.
Frequently Asked Questions
How do I find my factory axle ratio?
Check the driver’s door jamb sticker (look for an “AXLE” code), the glove box RPO sticker on GM vehicles (codes like GU6 = 3.42, GT4 = 3.73), or the tag bolted to the differential cover. As a last resort, jack up the rear, mark the driveshaft and a tire, and count driveshaft rotations per one tire rotation.
Numerically higher vs. lower — which is better for towing?
Numerically higher (4.10 vs. 3.42) multiplies torque to the wheels, so the engine works less hard pulling a load — better for towing and bigger tires, at the cost of higher cruise RPM and worse empty fuel economy. Numerically lower is the highway-economy choice. For regular towing, most people go one step higher than stock.
What does a re-gear cost?
Expect roughly $1,200–$2,000 per axle at a shop, including parts (ring and pinion set, bearings, seals) and labor. A 4×4 needs both axles done, so budget roughly double. It’s skilled work — incorrect pinion depth or backlash will whine or fail, so this isn’t a first-timer driveway job.
Can I run different ratios front and rear?
No — not on a 4×4. The front and rear driveshafts turn at the same speed through the transfer case, so mismatched ratios fight each other and break things. Front and rear must match (some full-time 4WD systems tolerate a 1% stagger from the factory, but never mix intentionally).
How do I verify the ratio after installation?
Jack up the rear axle, put the transmission in neutral, and mark the driveshaft and one tire. With an open differential, rotate the tire exactly two full turns while holding the opposite tire still (or rotate both tires together one full turn) — the number of driveshaft rotations equals your ratio, so 3.73 turns means 3.73 gears. Then take it on the highway: your cruise RPM at a known speed should match the calculator’s prediction for the new ratio.