Gear Ratio Calculator

REGEAR ANALYSIS

Find the right differential gear ratio after changing tire size.
Compare stock, ideal, and performance gearing to understand acceleration, towing, fuel economy, and whether a regear is worth the cost.

Current Setup

Current Tire Diameter

inches

Current Axle Gear Ratio

Transmission Top Gear(Optional)

Transfer Case Low Range(Optional)

New Setup

New Tire Diameter

inches

Account for tire squat (~3%)

Effective Tire Diameter33.95 inchesReduces effective diameter by 3%. Tires compress under vehicle weight, reducing effective rolling diameter by roughly 2–4%.

Results Summary

Ideal Gear Ratio

Best balance for daily driving

4.08

Closest available gear ratio4.10

Performance Ratio

For towing, crawling & heavy loads

4.30

Closest available gear ratio4.30

Keep Current Ratio

If you keep stock gears

3.73

Effective Ratio (with squat)3.41

Gearing VerdictWorth RegearingThe larger tire takes a real bite out of your effective gearing. Acceleration, towing, and throttle response all suffer enough that a regear is worth the money.

AccelerationRoughly 9% less torque multiplication off the line.
Fuel EconomyMore downshifts and engine load can lower real-world mpg until regeared.
TowingLess low-end torque makes towing and grades harder in the higher gears.
Highway DrivingCruise RPM drops about 174 at the same speed on stock gears.
Off-RoadBigger tires raise effective crawl gearing — deeper gears restore low-speed control.

Recommendation

You lose about 8.7% effective gearing. A regear to 4.10 restores daily response, or 4.30 for towing and trails.

Compare Tire Sizes

Gearing Comparison (With 35" Tires)

Keep Current (3.73)Effective: 3.41

3.41

Ideal (4.10)Effective: 3.74

3.74

Performance (4.30)Effective: 3.93

3.93

LowerOverall Gearing (Effective Ratio)Higher

Setup	Acceleration	Towing	Fuel Economy	Highway RPM
Keep Current (3.73)
Ideal (4.10)
Performance (4.30)

Poor

Fair

Good

Very Good

Excellent

Before Installing Bigger Tires

Switching from 31" to 35" tires with 3.73 gears will make your vehicle behave approximately like changing from 3.73 gears to 3.41 gears.

Expect:

Slower launches & acceleration
Weaker towing & hill climbing
Lower engine RPM at highway speeds
More transmission downshifts

Stock Setup

31" Tires3.73 Gears

After 35" Tires (Stock 3.73 Gears)

Effective Like3.41 Gears

Effective Gear Ratio Explained

When you increase tire size without changing gears, your effective (overall) ratio gets taller.

Effective Ratio(Overall)

Original Gear Ratio(Axle Ratio)Original Tire Diameter(Loaded)New Tire Diameter(Effective)

3.733133.95

3.41

Expected Regear Cost

Typical pricing by how you get the work done.

DIY$300–700parts only

Ring & pinion + install kit
Your own time & tools
Setup experience required

Most CommonProfessional Shop$800–2,000per axle

Parts, labor & precision setup
Pattern & backlash dialed in
Warranty on the work

Complete Front + Rear$1,500–3,500+both axles

4WD / AWD dual-diff regear
Optional lockers add cost
Best long-term drivability

Estimates only. Actual cost varies by vehicle, axle type, gear brand, locker installation, and regional labor rates. Always get a local quote before budgeting.

Gearing Comparison Table

Setup	Tire Diameter (Effective)	Axle Gear Ratio	Effective Gear Ratio (Overall)	Change from Stock	Acceleration	Towing	Cruising RPM
Stock (Current)	31.00"	3.73	3.73	—	Good	Fair	Baseline
New Tires (Stock Gears)	33.95"	3.73	3.41	-8.7%	Fair	Poor	-8.7% (Lower)
Recommended (Ideal)	33.95"	4.10	3.74	+0.4%	Fair	Fair	+0.4% (Higher)
Performance	33.95"	4.30	3.93	+5.3%	Good	Good	+5.3% (Higher)

Stock (Current)

Tire Diameter31.00"

Axle Gear3.73

Effective Ratio3.73

Change from Stock—

AccelerationGood

TowingFair

Cruising RPMBaseline

New Tires (Stock Gears)

Tire Diameter33.95"

Axle Gear3.73

Effective Ratio3.41

Change from Stock-8.7%

AccelerationFair

TowingPoor

Cruising RPM-8.7% (Lower)

Recommended (Ideal)

Tire Diameter33.95"

Axle Gear4.10

Effective Ratio3.74

Change from Stock+0.4%

AccelerationFair

TowingFair

Cruising RPM+0.4% (Higher)

Performance

Tire Diameter33.95"

Axle Gear4.30

Effective Ratio3.93

Change from Stock+5.3%

AccelerationGood

TowingGood

Cruising RPM+5.3% (Higher)

How Tire Size Changes Affect Your Gearing

When you install larger tires, you change far more than ground clearance and looks. A taller tire travels a greater distance with every rotation of the axle, which reduces the torque delivered to the ground for a given engine output. Mechanically, this behaves exactly like switching to a numerically lower differential gear — your axle still reads 3.73:1, but it performs like something taller.

The effective gear ratio is the number that actually matters for how the vehicle feels. It is calculated by multiplying your stock gear ratio by the ratio of stock tire diameter to new tire diameter. The further your new tire diameter is from stock, the more effective gearing you lose, and the more your acceleration, towing capability, and throttle response degrade.

Restoring lost performance is the job of a regear. The ideal gear ratio is found by multiplying your stock gear by the ratio of new tire diameter to stock diameter — it returns your effective gearing to its original value. Because differential gears are only made in fixed steps, the calculator snaps the ideal value to the closest commonly available ratio so you can shop realistically.

Drivers who tow, crawl, or carry heavy loads often choose a gear slightly deeper than the daily-driving ideal. A deeper ratio multiplies torque further, keeps the engine in its powerband under load, lowers transmission temperatures, and improves low-speed control off-road. The trade-off is marginally higher cruise RPM and a small fuel-economy penalty at steady highway speeds.

Cruise RPM is the other half of the equation. Engine speed at a given road speed scales with both gear ratio and vehicle speed and falls as tire diameter grows. Larger tires drop your highway RPM with stock gears, which can push the engine out of its efficient operating range and trigger frequent downshifts. Matching your gear ratio to your tire size and driving style keeps RPM where the engine is happiest.

Finally, remember that tires deform under vehicle weight. The loaded, rolling diameter is typically 2–4% smaller than the advertised size, so an effective-diameter correction produces more realistic gearing, speedometer, and RPM numbers than using the nominal tire size alone.

Frequently Asked Questions

Do I need new gears after increasing tire size?

It depends on how much bigger the tire is and how you drive. A small jump of 3–5% in diameter usually feels fine on stock gears. Once you lose more than about 7–10% of effective gearing — common when going up 3+ inches in tire size — acceleration, towing, and highway response suffer enough that a regear is worth it. This calculator shows your exact effective gearing loss so you can decide.

Will bigger tires reduce acceleration?

Yes. A larger tire increases the distance traveled per axle revolution, which reduces torque multiplication at the contact patch. The effect is the same as installing a numerically lower (taller) gear. The bigger the tire relative to stock, the more noticeable the loss — especially from a stop, while towing, and on grades. Regearing back to your original effective ratio restores stock-like acceleration.

How much does a gear swap cost?

Parts for a ring-and-pinion plus install kit typically run $300–800 per axle. Professional installation and setup adds $500–1,200 per axle because gear setup is precision work. A 2WD truck with one driven axle lands near the lower end, while a 4WD or AWD vehicle with two differentials can total $1,500–3,500+ once both axles are done. Doing it yourself saves labor but requires specialized tools and experience.

What's the difference between actual and effective gear ratio?

Your actual (or numerical) gear ratio is the physical ring-and-pinion ratio in the axle, such as 3.73:1. The effective gear ratio is how that ratio behaves once tire size changes. Fitting a taller tire makes a 3.73 axle perform like a numerically lower ratio because the wheel travels farther per turn. Effective gearing is what you actually feel — it drives acceleration, towing, and cruise RPM.

Will larger tires hurt fuel economy?

Often, yes — at least until you regear. Bigger, heavier tires add rotating mass and reduce effective gearing, so the engine works harder and downshifts more, especially while towing or climbing. Highway cruise RPM drops, which can help economy slightly in isolation, but the added weight and aerodynamic drag of taller tires usually outweighs that. Restoring effective gearing with a regear typically recovers most of the lost economy.

Should I regear for towing?

If you tow regularly on larger tires, regearing is one of the best upgrades you can make. Larger tires reduce low-end torque exactly when you need it most, forcing the transmission to hold lower gears and run hotter. A deeper (numerically higher) gear restores torque multiplication, keeps the engine in its powerband, reduces heat, and improves control on grades. For towing builds, choosing a slightly deeper gear than the daily-driving ideal is common.