How Do Bigger Tires Change RPM and Gear Ratio?

Installing taller tires lowers engine RPM at highway speeds and creates a numerically lower effective gear ratio, which reduces acceleration. Because a taller tire has a larger circumference, each turn covers more distance along the road. The tire completes fewer revolutions per mile, so your engine spins more slowly for the same vehicle speed.

Why do bigger tires lower engine RPM?

In the examples below, engine speed is calculated from wheel speed, the axle ratio, and the transmission ratio, so a change in tire rotation changes the calculated engine speed.

The formula calculates revolutions per mile by dividing 63,360 inches (one mile) by tire circumference. Because larger tires have greater circumference, the formula calculates fewer revolutions per mile. In the 65 mph example below, fewer wheel rotations give a lower calculated engine RPM.

How do taller tires change your effective gear ratio?

The axle ratio (final drive ratio) is set for the factory tire diameter. When you install taller tires, the effective ratio becomes numerically lower, as if the axle gears had changed.

According to OnAllCylinders, the new effective ratio equals the old ratio multiplied by old diameter divided by new diameter.

In an example published by OnAllCylinders, tire diameter increases from 26 inches to 28 inches with a 3.55 axle ratio. Under the OnAllCylinders formula, that change reduces the calculated effective gear ratio to 3.30, which OnAllCylinders describes as reduced acceleration.

To keep the original performance, OnAllCylinders divides 28 inches by 26 inches and multiplies by 3.55, calculating a required 3.82 ratio. Changing axle gears is called re-gearing, which you should discuss with a professional automotive shop.

How do you calculate RPM and gear ratio changes?

1. Find tire diameter. Calculate diameter with the nominal sizing formula.
2. Find revolutions per mile. Divide 63,360 inches by tire circumference.
3. Find wheel and engine RPM. Multiply revolutions per mile by speed in mph and divide by 60 to get wheel RPM, then multiply by the axle ratio and the transmission ratio to get engine RPM.
4. Find effective ratio. Multiply axle ratio by old diameter divided by new diameter.

Example: 265/70R17 to 285/70R17

1. Original diameter: the formula calculates that 265/70R17 has an overall diameter of 31.61 inches.
2. New diameter: the formula calculates that 285/70R17 has an overall diameter of 32.71 inches.
3. Revolutions per mile: the formula calculates 638.1 revolutions per mile for 265/70R17 and 616.6 for 285/70R17 (-3.4%).
4. Wheel rotation: at 65 mph, the formula calculates 691.3 wheel rpm for 265/70R17 versus 668.0 for 285/70R17.
5. Engine RPM: assume a 3.73 axle ratio and 1:1 transmission ratio in this hypothetical example. The formula calculates ~2,578 engine rpm on 265/70R17 versus ~2,492 on 285/70R17.
6. Effective ratio: multiplying 3.73 by 31.61 divided by 32.71 gives a calculated effective ratio of 3.60 using the OnAllCylinders formula.

Compare these sizes in our tire size calculator.

Which values change with larger tire sizes?

The table below lists calculated nominal dimensions and published specifications.

The calculated rows come from the nominal size and the formula. The Tire Rack row is a published specification for one specific tire, the LT285/70R17 KO2, which lists 32.8 inches and 635 revolutions per mile. These are two different kinds of figure: the formula gives 32.71 inches and 616.6 revolutions per mile for the nominal 285/70R17, so the published revolutions per mile is about 3% higher than the calculated one. Applying the same formula to Tire Rack's 32.8 inches gives 614.9 revolutions per mile, which differs from the published value, so the published figures do not follow from each other by this formula. Do not mix the two kinds of figure in one calculation. Use either the nominal values or the published values for both tires.

What else should you check before upsizing tires?

• Tire placard. NHTSA notes that the vehicle's tire size appears on the Tire and Loading Information label on the driver's door edge or in the owner's manual.
• Sidewall markings. Review how to read a tire sidewall to confirm load index and speed ratings.
• Alternative sizes. Explore dimensions with the tire size alternatives tool before changing gears.
• Fitment and gear work. This page calculates geometry only and does not show whether a size fits your vehicle. Consult an automotive repair shop for fitment and gear work.

Frequently asked questions

Will bigger tires make my vehicle feel slower?

OnAllCylinders describes reduced acceleration when tire diameter grows without a gear change. In its example, moving from 26 inches to 28 inches lowers the effective ratio from 3.55 to a calculated 3.30.

Do you have to re-gear after installing taller tires?

The sources do not say when re-gearing is needed. OnAllCylinders shows the arithmetic: multiplying the old ratio by new diameter divided by old diameter gives the ratio that keeps the original effective ratio (3.82 in its 26 to 28 inch example). Discuss gear options with an automotive professional.

Where do you find your vehicle's factory tire size?

NHTSA states that the correct tire size for your vehicle appears on the Tire and Loading Information label on the driver's door edge or in the owner's manual. Check that label before selecting new tires.

Sources

• OnAllCylinders: Final Drive Ratio Calculation
• Tire Rack: BFGoodrich KO2 LT285/70R17 Specs
• NHTSA: Tire Safety and Maintenance