Tire Size Calculator
Compare two tire sizes side by side. See overall diameter, sidewall height, circumference, revolutions per mile, and exactly how much your speedometer will be off, all instantly, with no signup required.
Tire 1 (e.g. current / stock)
Width / aspect ratio R rim diameter
Tire 2 (e.g. new / candidate)
Width / aspect ratio R rim diameter
Side-by-side comparison
| Metric | 265/70R17 | 285/65R18 | Difference |
|---|---|---|---|
| Overall diameter | 31.61" | 32.59" | +0.98" |
| Diameter (mm) | 803mm | 828mm | +25mm |
| Section width | 265mm | 285mm | +20mm |
| Sidewall height | 185.5mm | 185.3mm | -0.2mm |
| Circumference | 99.29" | 102.37" | +3.08" |
| Revolutions per mile | 638 | 619 | -19 |
The new size is +3.1% larger. At an actual 60 mph your speedometer will read about 58.2 mph.
This exceeds the common 3% safe threshold. Odometer accuracy, ABS/traction control calibration, and fender clearance can all be affected. Confirm clearance before buying.
Tire 2 is 3.1% larger (+0.98" overall diameter). That is beyond the common 3% safe threshold, so check fender and suspension clearance before buying.
The new size is +3.1% larger. At an actual 60 mph your speedometer (calibrated to Tire 1) will read about 58.2 mph on Tire 2.
Tire 2 turns 19 fewer times per mile, so it covers more ground per rotation (taller effective gearing).
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Guide
How to use this calculator
1. Enter both sizes
Type each tire spec in the standard format: width / aspect ratio R rim diameter, for example 265/70R17. You can find this printed on the sidewall of any tire, or in your vehicle's door jamb sticker.
2. Read the comparison table
The calculator instantly computes overall diameter, sidewall height, circumference, and revolutions per mile for each size, then shows the exact difference between them.
3. Check the verdict
A plain-English summary tells you the percentage size difference and what your speedometer will read at an actual 60 mph. Differences over 3% are flagged so you know to double-check clearance.
Guide
How to read a tire size
What each number in 265/70R17 actually means, and where to find it on your tire or door jamb.
Guide
Plus-sizing wheels
How much bigger you can safely go on wheel diameter while keeping your tire diameter close to stock.
Tool
Full wheel fitment calculator
Check offset, backspacing, and fender clearance in addition to tire size, all in one tool.
Next step
Now find wheels that fit your car
Vehicle hub
Tesla Model Y
See wheels confirmed to fit, filtered and ready to browse.
Vehicle hub
Ford F-150
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Vehicle hub
Toyota Tacoma
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Vehicle hub
Toyota 4Runner
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Vehicle hub
Jeep Wrangler
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Vehicle hub
Chevrolet Silverado 1500
See wheels confirmed to fit, filtered and ready to browse.
FAQ
Common questions
- How do I compare two tire sizes?
- Enter both sizes in standard format, like 265/70R17, into the calculator above. It parses the section width, aspect ratio, and rim diameter for each size, then computes overall diameter, circumference, and revolutions per mile so you can see exactly how the two sizes differ before you buy.
- How much bigger can I safely go with a tire size?
- A common rule of thumb is to stay within about 3% of your original overall diameter. Beyond that, you risk speedometer and odometer error, altered ABS and traction control calibration, and potential rub against the fender or suspension components. The calculator flags any comparison that exceeds this 3% threshold.
- How does tire size affect my speedometer?
- Your speedometer is calibrated to your original tire's rolling circumference. A larger tire covers more distance per rotation, so the wheel spins slower than expected for a given road speed, meaning the speedometer reads lower than your actual speed. A smaller tire does the opposite: it spins faster, so the speedometer reads higher than actual. The calculator shows the indicated speed you would see at an actual 60 mph.
- What does revolutions per mile mean?
- Revolutions per mile is how many times a tire fully rotates to cover one mile, calculated as 63,360 inches (one mile) divided by the tire's circumference in inches. A larger-diameter tire has a longer circumference, so it needs fewer revolutions to cover a mile. This number is also what drives speedometer and odometer accuracy, and why gear ratio and tire size are linked in trucks and off-road builds.
- What is the difference between tire diameter and sidewall height?
- Sidewall height is just the tire's side wall, from the rim edge to the tread, calculated as the section width multiplied by the aspect ratio percentage. Overall diameter is the full height of the tire from the ground to the top, equal to the rim diameter plus two sidewall heights (one on each side of the rim). Overall diameter is what determines speedometer accuracy and fitment clearance.
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