Current gears with new tires
3.30 effective ratio
11.4% taller than the original setup
Lower RPM and low-speed multiplication at the same road speed
Gear Ratio Calculator
REGEAR ANALYSISFind the axle ratio that restores your original effective gearing after a tire size change.
31 in Current Tire35 in New Tire3.73 Current Axle Ratio
Enter Your Setup
Current
in
ratio
Enter a custom ratio or pick a common preset such as 3.73 or 4.10.
New
in
Use actual mounted tire diameter when available.
Exact stock-like target
4.21
Restores the original geometric gearing relationship
Changing from 31-inch to 35-inch tires with 3.73 gears produces an exact stock-like target of 4.21.
Compare the gearing options
Markers show the relative numerical position of each ratio.
- Current effective11.4% taller than original3.30
- Nearby example2.4% taller than original4.11
- Exact targetMatches original4.21
- Nearby example2.1% deeper than original4.30
- Deeper target5.0% deeper than original4.42
Nearby common ratio examples
Confirm availability for your specific axle and differential.
Optional deeper target
4.42 target
5% deeper than the stock-like target
Higher RPM and low-speed multiplication
Advanced RPM & Crawl AnalysisOptional speed, transmission and low-range inputs
mph
Used to estimate engine RPM at a selected road speed.
Enter the transmission ratio used at your selected cruising speed.
Required to calculate overall crawl ratio.
Required for four-wheel-drive low-range crawl-ratio calculations.
User-selected mathematical target relative to the exact stock-like target — not a vehicle-specific recommendation.
What Changes With the New Tires?
Moving from 31-inch to 35-inch tires makes the effective gearing 11.4% taller when the existing 3.73 gears are retained.
Effective gearing
11.4% taller
Current gears behave like a 3.30 ratio
Engine RPM
Approximately 11.4% lower
At the same road speed
Low-speed multiplication
Approximately 11.4% lower
Relative to the original setup
Larger tire
More distance per revolution
Taller effective gearing
Keeping the current axle ratio means lower RPM and less low-speed multiplication at the same road speed.
These figures describe geometric tire-diameter and axle-ratio relationships only. They are not vehicle-specific fitment or availability claims.
Setup Comparison
Geometric comparison of axle and effective ratios. Nearby examples are common reference values, not availability claims.
Original setup
- Axle ratio
- 3.73
- Effective ratio
- 3.73
- Difference
- 0% / Original
Original reference
New tires with current gears
- Axle ratio
- 3.73
- Effective ratio
- 3.30
- Difference
- 11.4% taller
Taller than original
New tires with exact stock-like targetExact target
- Axle ratio
- 4.21
- Effective ratio
- 3.73
- Difference
- 0% difference
Original gearing restored mathematically
New tires with nearest common-ratio example
- Axle ratio
- 4.30
- Effective ratio
- 3.81
- Difference
- 2.1% deeper
Slightly deeper
New tires with optional deeper target
- Axle ratio
- 4.42
- Effective ratio
- 3.92
- Difference
- 5.0% deeper
Deeper target
How the calculation worksView the effective-ratio and stock-like-target formulas
Effective ratio with new tires
How the current gears behave with the new tires
3.73 times 31 divided by 35 equals 3.30.
Current axle ratio × current tire diameter ÷ new tire diameter.
Stock-like target ratio
Ratio that restores the original geometric relationship
3.73 times 35 divided by 31 equals 4.21.
Current axle ratio × new tire diameter ÷ current tire diameter.
Use actual mounted tire diameters for the most accurate comparison. Calculations use geometric tire-diameter and axle-ratio relationships only.
Typical US Regear CostView broad US planning estimates and cost factors
- Parts only$300–700typical ring-and-pinion + install kit
Does not include specialised tools, setup gauges or labour.
- Professional installation per axle$800–2,000parts and labour, one axle
Includes precision setup such as pattern and backlash where quoted by the shop.
- Front and rear axle regear$1,500–3,500+both axles, US shops
Common total range when a 4WD or AWD vehicle needs matching front and rear ratios.
Additional bearings, lockers or differential work: Bearings, seals, lockers, carrier upgrades and damaged differential repair are priced separately.
These figures are not quotes and do not apply as-is in the UK, Canada, Australia or New Zealand.
How Tire Size Changes Effective Gearing
Why larger tires make gearing taller
A larger tire travels farther with each wheel revolution, so the same axle ratio delivers taller effective gearing.
Engine RPM and low-speed wheel multiplication both drop at a given road speed.
The stamped numerical axle ratio itself does not change.
Diameter change alters effective gearing without rewriting the axle stamp.
Restoring stock-like gearing
The exact target scales the current axle ratio by the tire-diameter change.
That restores the original geometric relationship between engine speed, axle rotation and tire travel.
Using actual mounted tire diameter improves accuracy.
Stock-like targets restore geometry — not a vehicle-specific recommendation.
Choosing a deeper ratio
A numerically higher ratio raises engine RPM at a given road speed and increases low-speed multiplication.
The best final ratio still depends on the vehicle and intended use.
Availability depends on the axle and differential.
Deeper is a mathematical bias option, not a universal upgrade.
What the calculator cannot predict
It does not know engine torque curves, shift strategy, torque-converter behaviour or drivetrain losses.
Vehicle weight, aerodynamics, terrain, towing load and driving style are outside the geometric model.
It also cannot confirm axle-ratio availability or predict actual fuel economy.
Use the numbers as geometry — verify parts and drivetrain with a specialist.
Frequently Asked Questions
What gear ratio do I need for larger tires?
Scale your current axle ratio by new tire diameter ÷ current tire diameter to get an exact stock-like target. That restores approximately the same geometric relationship as the original setup. Compare nearby manufactured ratios and confirm what your axle and differential support.
How do larger tires affect effective gear ratio?
A larger tire travels farther per wheel revolution, so the same axle ratio delivers taller effective gearing. Engine RPM and low-speed wheel multiplication both drop at a given road speed even though the stamped ring-and-pinion ratio is unchanged.
Does changing tire size change engine RPM?
Yes, at the same road speed and transmission gear. Larger tires generally lower engine RPM with the current axle ratio; smaller tires raise it. Estimated RPM also depends on top-gear ratio and the speed you choose for comparison.
Is the exact calculated ratio always available?
The exact mathematical target may not be manufactured for every axle. Compare nearby ratios and confirm compatibility with your axle and differential.
Should I use advertised or measured tire diameter?
Use the same measurement type for both tires. Advertised sizes can differ from mounted diameter under load. Actual mounted measurements improve accuracy for effective gearing and stock-like targets.
Do I need to regear both axles on a four-wheel-drive vehicle?
Front and rear axle ratios normally need to match when both axles are mechanically engaged. Confirm the requirements for your drivetrain before changing ratios.