Gear Ratio Calculator for Tire Size
- Gearing change
- %
- Result
- Speed at 3000 RPM (new tires)
- mph
- Result
- Current tire diameter
- 31 in
- You entered
A gear ratio calculator for tire size works out the effective final drive ratio (or the speed at a given RPM) after you change tire diameter, ring-and-pinion gears, or transmission gearing. You enter your current tire diameter, new tire diameter, and gear ratio (or your RPM and gear), and it returns the new effective ratio, the percentage change in gearing, and the speed difference at the same engine speed. This lets you predict how a tire swap will affect acceleration, fuel economy, and speedometer accuracy before you spend money on parts.
Formula
The core formula for the effective gear ratio after a tire size change is:
Effective Ratio = (Old Tire Diameter / New Tire Diameter) × Current Ratio
Where:
- Old Tire Diameter (inches or mm) – the diameter of the tire you are replacing.
- New Tire Diameter (inches or mm) – the diameter of the tire you plan to install.
- Current Ratio – the existing final drive (axle) ratio, e.g., 3.73:1.
The result is the axle ratio that would give you the same effective gearing with the new tires. If the effective ratio is lower (numerically smaller), you have taller gearing (more top speed, less acceleration). If it is higher (numerically larger), you have shorter gearing (more acceleration, lower top speed).
Worked Example
Using the calculator defaults: old tire diameter = 31 in, new tire diameter = 33 in, current ratio = 3.73.
- Divide old by new: 31 / 33 = 0.9394
- Multiply by current ratio: 0.9394 × 3.73 = 3.50
The effective gear ratio is 3.50. This means that with 33-inch tires, your 3.73 gears behave like 3.50 gears. The gearing change is (33/31 – 1) × 100 = 6.5% taller. At 3000 RPM in a gear with a 1:1 ratio, the speed with the new tires is approximately (33 × π × 3000) / (3.73 × 1056) = 78.9 mph (the calculator shows 78.9).
Why Tire Size Changes Gearing
Tire diameter directly affects the distance traveled per wheel revolution. A larger tire covers more ground per revolution, so for a given engine RPM and transmission gear, the vehicle moves faster. However, the engine still turns the same number of times per minute, so the effective torque at the wheels decreases. This is why larger tires feel “sluggish� off the line unless you compensate with lower (numerically higher) axle gears.
Conversely, smaller tires make the engine turn more RPM at the same speed, which can improve acceleration but reduce fuel economy on the highway and raise engine wear.
How to Choose a New Gear Ratio
To restore the original performance after upsizing tires, you want a new ratio that matches the old effective ratio. Use this formula:
New Ratio = Current Ratio × (New Tire Diameter / Old Tire Diameter)
For the example above: 3.73 × (33/31) = 3.97. So you would need approximately 4.10 gears (a common available ratio) to get close to the original performance. Many off-roaders choose 4.10 or 4.56 for 33-inch tires depending on engine power and driving style.
If you are not regearing, the calculator tells you what to expect: a 6.5% loss in torque at the wheels and a 6.5% speedometer error (you will be going faster than the speedometer reads).
Speedometer and Odometer Error
Larger tires cause the speedometer to read lower than actual speed because the vehicle travels farther per revolution. The error is proportional to the diameter change. For example, going from 31s to 33s (a 6.5% increase) means at an indicated 60 mph you are actually going 63.9 mph. Odometer readings will also undercount by the same percentage, which can affect fuel economy calculations and warranty mileage.
Many modern vehicles can be recalibrated with a programmer, but if you have a mechanical speedometer, you may need to change the speedometer gear in the transmission.
Common Mistakes and Misconceptions
- Using tire width instead of diameter: The formula only uses diameter. Width affects rolling resistance and sidewall flex, but not the gear ratio calculation.
- Mixing units: Always use the same unit (inches or mm) for both tire diameters. If you have metric sizes, convert to inches (1 inch = 25.4 mm) or use a tire size calculator first.
- Ignoring tire inflation: Underinflated tires have a slightly smaller effective diameter. The difference is small but can matter for precise calculations.
- Assuming all 33-inch tires are exactly 33 inches: Actual diameters vary by brand and model. Check the manufacturer’s spec for the true diameter.
- Forgetting transmission gear ratios: The calculator gives the effective axle ratio, but if you want speed at a specific RPM, you need to include the transmission gear ratio (e.g., 1:1 in high gear, or 0.67 overdrive). The formula for speed is: (RPM × Tire Diameter) / (Gear Ratio × Axle Ratio × 336) for mph (or × 0.0114 for km/h).
Limitations of the Formula
The effective ratio formula is a simplified model. It assumes a rigid tire and no slip. In reality, tires deform under load, and the rolling radius is slightly less than the unloaded radius. Also, the formula does not account for aerodynamic drag, rolling resistance, or drivetrain losses, which affect actual performance. For most purposes, the error is under 2% and acceptable for planning.
For precise speed calculations, use the tire’s “revolutions per mile� (rev/mile) figure from the manufacturer, which accounts for loaded radius. The formula then becomes: Speed (mph) = (RPM × 60) / (Gear Ratio × Axle Ratio × Rev/mile).
Frequently Asked Questions
How do I find my tire diameter?
Look on the sidewall for a size like P265/70R17. The first number is width in mm, the second is aspect ratio (sidewall height as a percentage of width), and the third is rim diameter in inches. Diameter = (2 × width × aspect ratio / 2540) + rim diameter. For P265/70R17: (2 × 265 × 0.70 / 25.4) + 17 = 31.6 inches.
Will larger tires hurt my fuel economy?
Yes, generally. Larger tires increase rolling resistance and aerodynamic drag, and if you do not regear, the engine runs at lower RPM on the highway, which can actually improve fuel economy in some cases, but the extra weight and drag usually offset that. Expect a 1-3 mpg drop for a 2-inch diameter increase.
Do I need to recalibrate my speedometer after changing tire size?
If the diameter change is more than 3-5%, yes, to avoid inaccurate speed and odometer readings. Many vehicles can be recalibrated with an OBD-II programmer, or you can install a speedometer gear (older vehicles).
What gear ratio should I use for 35-inch tires?
It depends on your vehicle and engine. A common recommendation for jeeps and trucks is 4.56 for 35s, but for lighter vehicles or more powerful engines, 4.10 may suffice. Use the formula: current ratio × (35/old diameter) to find the equivalent.
Can I use a gear ratio calculator for bicycles?
Yes, the same principle applies. For bicycles, you use chainring teeth, cassette teeth, and wheel diameter to calculate gear inches or development. The formula is: gear inches = (chainring / cog) × wheel diameter in inches.