Calculating Gear Ratio With Tire Size: What Changes and How to Compute It

To keep the same performance after changing tire size, multiply your original axle ratio by the ratio of new tire diameter to old diameter. In symbols: required axle ratio = original axle ratio × (new tire diameter ÷ original tire diameter). If you don’t regear, your effective axle ratio becomes original axle ratio × (original tire diameter ÷ new tire diameter). This article explains why that works, how to get tire diameter from size markings, and how to estimate RPM, speed, and speedometer error.

Why Tire Size Affects Gear Ratio

Bigger tires travel farther per revolution, which reduces the mechanical “leverage” of your final drive. The drivetrain sees this as a taller (numerically smaller) overall ratio. That can soften acceleration, raise clutch load, and lower cruising RPM; smaller tires do the opposite. Matching axle gears to tire diameter restores the original overall ratio and the feel you’re used to.

From Tire Size to Tire Diameter

Reading the size on the sidewall

Tire sizing appears in two common formats. Understanding these lets you convert to overall diameter in inches, which you’ll use in the formulas below.

• P-metric (e.g., 285/70R17): section width in millimeters, aspect ratio (sidewall as a percent of width), and wheel diameter in inches.
• Flotation (e.g., 33×12.50R15): overall diameter first (33 inches), section width (12.50 inches), then wheel diameter (15 inches).

P-metric sizes require a quick calculation to get overall diameter; flotation sizes already include the diameter as the first number.

Converting P-metric to overall diameter

Use the following steps to convert a P-metric tire size to overall diameter in inches.

1. Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100.
2. Sidewall height (in) = sidewall height (mm) ÷ 25.4.
3. Overall diameter (in) = wheel diameter (in) + 2 × sidewall height (in).

Example: 285/70R17 → sidewall height = 285 × 0.70 = 199.5 mm = 7.85 in; overall diameter = 17 + 2 × 7.85 ≈ 32.7 inches.

Key formulas you’ll use

These formulas relate tire diameter, axle gears, transmission ratio, speed, and engine RPM. They’re commonly used by driveline shops and gear calculators.

• Effective axle ratio after a tire change (no regear): effective ratio = original axle ratio × (original tire diameter ÷ new tire diameter).
• Required axle ratio to restore stock feel: new axle ratio = original axle ratio × (new tire diameter ÷ original tire diameter).
• Engine RPM at a given speed: RPM ≈ (mph × transmission gear ratio × axle ratio × 336) ÷ tire diameter (inches). Include overdrive (e.g., 0.75) if applicable.
• Speedometer error factor when tires change (no recalibration): actual speed ≈ indicated speed × (new tire diameter ÷ original tire diameter).

If you know any three variables, you can estimate the fourth. These relationships let you plan a regear or predict highway RPM and speedometer error.

Worked examples

Below are common scenarios that show how to apply the formulas to real-world changes.

1. Regearing for larger tires to maintain stock performance:

   Assume stock: 265/70R17 (≈31.6 in) with 3.73 axle ratio; new tires: 35 in.

   New axle ratio = 3.73 × (35 ÷ 31.6) = 3.73 × 1.107 ≈ 4.13.

   Closest common gear set: 4.10 (slightly shorter than 4.13, very close in practice).

2. What if you don’t regear with 35s?

   Effective axle ratio = 4.10 × (31.6 ÷ 35) ≈ 4.10 × 0.903 ≈ 3.70.

   You’ve effectively turned a 4.10 into a 3.70, so launches feel softer and towing performance drops.

3. Highway RPM estimate:

   At 70 mph, 35-in tire, 4.10 axle.

   In 1:1 gear: RPM ≈ (70 × 1.00 × 4.10 × 336) ÷ 35 ≈ 2,755 rpm.

   In 0.75 overdrive: RPM ≈ 2,755 × 0.75 ≈ 2,066 rpm.

These examples show how tire diameter shifts both the feel of the final drive and your cruising RPM, and how a regear can bring you back to baseline.

Finding your current axle ratio

If you’re not sure what gears you have now, there are several reliable ways to identify them.

• Build info: window sticker, manufacturer build sheet, or VIN decoder often lists the axle code and ratio.
• Axle tag or cover: many differentials have a metal tag or stamping with the ring and pinion tooth count (e.g., 41-11 → 41 ÷ 11 = 3.73).
• Driveshaft rotation method: with the vehicle safely lifted and in neutral, rotate a tire one full turn (open diff) and count driveshaft turns to estimate the ratio.
• Service records or prior owner notes: previous regears are sometimes documented by shops.

Confirming your current ratio avoids calculating from the wrong baseline and ensures you choose compatible front and rear ratios on 4x4s.

Common pitfalls and practical tips

When you plan a tire or gear change, keep these considerations in mind to avoid unpleasant surprises.

• Measured vs. labeled size: many “35-inch” tires measure closer to 34–34.8 inches unloaded and can grow slightly at speed; use a measured diameter for precision.
• Transmission gearing matters: aggressive overdrive ratios can drop RPM more than you expect; factor in your top-gear ratio when estimating highway RPM.
• Speedometer and ABS: modern vehicles may require electronic recalibration after tire or gear changes for accurate speed, odometer, and shift scheduling.
• Towing and terrain: frequent towing or steep grades may justify a slightly deeper (numerically higher) gear than the math suggests.
• 4×4 matching: front and rear axle ratios must match exactly; never mix ratios on a part-time 4WD.
• Break-in and setup: new ring and pinions need proper setup, break-in (typically gentle driving and cool-down cycles), and fresh gear oil after the initial period.

Getting these details right preserves drivetrain longevity and the driving characteristics you want, on- or off-road.

Summary

To figure gear ratio with tire size, convert both old and new tires to overall diameter and apply: required axle ratio = original axle ratio × (new diameter ÷ original diameter). Without regearing, your effective ratio becomes original axle ratio × (original diameter ÷ new diameter). Use RPM ≈ (mph × transmission ratio × axle ratio × 336) ÷ tire diameter to estimate cruising RPM, and correct your speedometer by the ratio of new to old diameter. With accurate tire measurements and your current axle ratio in hand, you can confidently plan a regear or predict how a tire change will affect performance.

How do you tell if you have 3.73 or 4.10 gears?

To identify whether you have 3.73 or 4.10 gears, check the differential housing for a metal tag, the vehicle’s door jamb or glove box for a build sheet or RPO code, or inspect the ring gear itself for stamped teeth numbers to calculate the ratio. Alternatively, you can lift the rear of the vehicle, mark a tire and the driveshaft, and count the driveshaft rotations for one tire rotation to determine the ratio.
 
This video demonstrates how to check the gear ratio by counting the number of teeth on the ring gear: 44sJL LIFE OFFROADYouTube · Nov 18, 2022
1. Check the Differential Tag or Vehicle Documentation 

• Differential Tag: Opens in new tabLook for a metal tag attached to a bolt on the differential cover; it often has the gear ratio printed on it. 
• Vehicle Documentation: Opens in new tabCheck your vehicle’s door jamb for a build sheet or RPO (Regular Production Option) code sticker, or review your original paperwork and window sticker. 

2. Use the Ring Gear Teeth Count Method (More Involved) 

1. Lift and Secure: Jack up the rear of the vehicle and secure it on jack stands, ensuring the wheels are off the ground. 
2. Find the Ring Gear: Remove the differential cover to expose the ring gear. 
3. Locate the Numbers: Look for numbers stamped on the ring gear. These numbers typically represent the teeth on the ring gear and the pinion gear (e.g., 41 and 11). 
4. Calculate the Ratio: Divide the ring gear’s teeth by the pinion gear’s teeth. 
   • Example: 41 teeth / 11 teeth = 3.727, which is a 3.73 ratio. 
   • Example: 41 teeth / 10 teeth = 4.10, which is a 4.10 ratio. 

3. Use the Driveshaft Rotation Method (Simple Check)

1. Lift and Secure: Jack up the rear of the vehicle and place it on jack stands. 
2. Mark a Tire: Place a piece of tape on the outside of one tire. 
3. Mark the Driveshaft: Place a piece of tape on the driveshaft. 
4. Rotate the Tire: Spin the marked tire one full revolution (360 degrees). 
5. Count Driveshaft Rotations: 
   • If the driveshaft rotated about 3 and 3/4 times, the ratio is likely 3.73. 
   • If the driveshaft rotated just over 4 times, the ratio is likely 4.10. 

This video shows how to determine the gear ratio without removing the differential cover: 48sBent Motorsports OffroadYouTube · Sep 22, 2023

What is a good gear ratio for 35 inch tires?

A good gear ratio for 35-inch tires is generally 4.56 or 4.88, depending on your vehicle’s engine, transmission, and intended use. 4.56 gears offer better fuel economy and a slightly lower cruising RPM for daily driving, while 4.88 gears provide more low-end torque and crawl control, making them ideal for off-roading and technical terrain. 
Factors to Consider

• Engine Power: More powerful engines with higher torque, such as a V8, may perform well with a lower numerical gear ratio, like 4.11s or 4.56s. Less powerful engines, like some inline-6 engines, will benefit more from higher numerically (steeper) ratios like 4.88s. 
• Transmission Type: Vehicles with an overdrive gear (like a 5-speed or automatic transmission with an overdrive gear) can better handle higher numerical gear ratios for highway cruising. 
• Vehicle Use:
  • Daily Driver/On-Road: If your vehicle is a daily driver and you spend a lot of time on the highway, 4.56 gears can provide a good balance of power and highway RPMs. 
  • Off-Road/Technical Terrain: For more extreme off-roading and technical crawling, 4.88 gears offer superior low-end torque, which is crucial for navigating difficult obstacles. 
• Axle Strength: Higher numerical gear ratios, like 5.13s, are considered steeper but can sometimes lead to weaker gears due to the smaller pinion. For 35-inch tires, 4.88s are often seen as a strong option, while 5.13s are a good choice if you plan to upgrade to larger tires in the future. 

Common Gear Ratio Recommendations

• 4.56 Gears: Opens in new tabA popular choice for daily driving with 35-inch tires, offering a good compromise between on-road performance and off-road capability. 
• 4.88 Gears: Opens in new tabExcellent for off-roading, providing better crawl ratios and more power for technical driving, though this comes at the cost of higher highway RPMs and lower fuel economy. 
• 4.10 or 4.11 Gears: Opens in new tabSome users may find these adequate, especially with a powerful engine and an overdrive transmission, though it may feel sluggish compared to steeper gears. 

What is the easiest way to determine gear ratio?

I have my buddy holding the other tire i start spinning this and I’m watching the other. One. That’s one full rotation right there.

What happens if you don’t regear with bigger tires?

What happens when you don’t regear? When a vehicle is equipped with larger tires, they require more effort to turn. This means your engine has to work harder to maintain speed, leading to a significant drop in acceleration, especially while climbing hills or towing heavy loads.