Effective Gear Ratio Calculator
Calculate the effective axle ratio after a tire size change. See how taller tires lower your real gearing for 3.21 to 4.56 axles.
Calculate gearing changes from tire diameter.
Effective Gear Ratio Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Effective Gear Ratio Calculator Works?
The Effective Gear Ratio Calculator works by scaling the installed axle ratio by the ratio of the original tire diameter to the new tire diameter. The tool uses 3 steps.
Enter the axle ratio
Type the installed ring-and-pinion ratio, for example 3.73.
Enter both tire diameters
Type the original and new sizes, for example 265/70 R17 (31.61 in) and 35x12.50R17 (35.0 in).
Read the effective ratio
The calculator returns 3.37, the ratio the truck now behaves like with the original tires.
Effective Gear Ratio Calculator Formula
The Effective Gear Ratio Calculator formula is Ratio_eff = Ratio_axle × (OD_orig ÷ OD_new).
R_eff = R_axle × OD_orig ÷ OD_newΔR% = (R_eff − R_axle) ÷ R_axle × 100R_total = R_gear × R_transfer × R_eff- R_axle
- Installed axle ratio
- OD_orig, OD_new
- Original and new tire diameter
- R_eff
- Effective ratio with new tires
Worked example
A truck with a 3.73 axle moves from 31.61 in to 35.0 in tires. The effective ratio is 3.73 × 31.61 ÷ 35.0 = 3.37, a 9.7% drop in effective gearing.
Effective Gear Ratio Calculator Conversion Table
The Effective Gear Ratio Calculator conversion table lists the effective ratio for 5 axle ratios and 6 tire heights, measured against an original 31.61-inch (803 mm) tire.
| New tire height | 3.21 axle | 3.55 axle | 3.73 axle | 4.10 axle | 4.56 axle |
|---|---|---|---|---|---|
| 31.61 in (803 mm) | 3.21 | 3.55 | 3.73 | 4.10 | 4.56 |
| 32.71 in (831 mm) | 3.10 | 3.43 | 3.60 | 3.96 | 4.41 |
| 33.00 in (838 mm) | 3.07 | 3.40 | 3.57 | 3.93 | 4.37 |
| 34.36 in (873 mm) | 2.95 | 3.27 | 3.43 | 3.77 | 4.20 |
| 35.00 in (889 mm) | 2.90 | 3.21 | 3.37 | 3.70 | 4.12 |
| 37.00 in (940 mm) | 2.74 | 3.03 | 3.19 | 3.50 | 3.90 |
What is Effective Gear Ratio Calculator?
An Effective Gear Ratio Calculator is a tool that shows the gearing a vehicle really has after a tire size change. A taller tire acts like a numerically lower axle ratio: the engine turns slower at a given speed and delivers less force at the road.
The effective ratio explains the sluggish feel, extra downshifting and towing struggle that follow a large tire upgrade.
Truck builders compare the effective ratio with the factory ratio to decide whether to install new ring-and-pinion gears.
What are the uses of Effective Gear Ratio Calculator?
There are 4 main uses of the Effective Gear Ratio Calculator, including regear decisions and towing checks.
Regear decisions
Measure how far large tires moved gearing from stock.
Towing checks
Compare effective gearing with the factory towing package ratio.
Crawl ratio planning
Combine effective axle ratio with transfer case low range.
Transmission tuning
Supply the effective ratio to shift-point tuning.