Gear Ratio Calculator: RPM and Regearing for Bigger Tires
Worked example
A truck on 265/70R17 tires with a 3.73 axle moves to 35x12.50R17. The 35 is about 10.7 percent taller than the 265/70R17 (31.6 in), so the equivalent ratio is 3.73 x 35 / 31.6 = about 4.13. The nearest common ratios are 3.73, 4.10 and 4.30. Which ones exist for your axle depends on the maker.
2923 rpm at 70 mph, 33x12.50R17, 4.10 axle, 1.00 gear
| Speed | Engine rpm |
|---|---|
| 40 mph | 1670 |
| 50 mph | 2088 |
| 60 mph | 2506 |
| 70 mph | 2923 |
| 80 mph | 3341 |
How to use this tool
- Find your numbers
Axle ratio is on the axle tag or in the build sheet. Use 1.00 for the transmission gear to get RPM in a direct-drive top gear, or enter the overdrive gear's ratio. - Read the RPM table
It shows engine speed from 40 to 80 mph for your tire, so you can see where the engine sits on the road. - Use the regear panel
Enter the ratio you have now, your old tire and your new tire. The equivalent ratio is the one that gives the old engine speed back.
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How it is calculated
- Engine rpm = mph x axle ratio x transmission gear ratio x 336.13 / tire diameter (in). 336.13 = 63,360 / (60 x pi).
- Equivalent ratio = old ratio x new diameter / old diameter.
Results are nominal. Real tires differ by maker and tread, and the maker's spec sheet lists the actual diameter.
Frequently asked questions
What is a gear ratio and where do I find mine?
A ring-and-pinion ratio is the number of ring gear teeth divided by the number of pinion teeth. Look for it on the axle tag, the build sheet or the owner's information, and confirm with a shop if the tag is missing.
How is engine RPM worked out from speed?
RPM = mph x axle ratio x transmission gear ratio x 336.13 / tire diameter in inches. The 336.13 is 63,360 inches per mile divided by 60 minutes and by pi.
Which transmission gear should I enter?
Enter the gear ratio you want to check. Use 1.00 for a direct top gear, or the overdrive ratio (for example 0.75) to see cruising RPM.
Why do bigger tires make a vehicle feel slower?
A taller tire acts like a taller gear: the engine turns fewer times per mile. One gear maker notes that regearing can win back drivability lost to bigger tires.
Can I pick any ratio the calculator shows?
No. The equivalent ratio is a target. The common ratios listed are examples, and which ratios exist for your axle depends on the gear maker. Check with the gear maker or your shop.
Does regearing fix speedometer error too?
Not by itself. The speedometer reads wheel speed, so a taller tire still changes it, and it usually needs recalibration. Use the speedometer calculator to see the size of the error.
Link to this tool
Your numbers are saved in the link, so anyone who opens it sees the same result.
Sources
Last checked 2026-11-12. Each link was opened and read on that date.
- Yukon Gear & Axle: Technical FAQs: Regearing can win back drivability lost to bigger tires; examples of common ratios.
- Yukon Gear & Axle: Gear ratio calculator: Ratio = ring gear teeth divided by pinion teeth.
This guide reflects independent editorial research, not first-hand product testing, professional mechanical training, or rescue certification. Some links on this page are Amazon Associates affiliate links; see our affiliate disclosure.