Free Tool
Tire Size Calculator
Find every alternate tire size for any wheel diameter, compare sizes side by side, and see exactly how a change affects your speedometer — then shop the size in one click.
Alternate Tire Size Finder
Enter your current tire size, then pick a wheel diameter to see every size that keeps your speedometer accurate (within 3% of original diameter) — including flotation sizes like 33x12.50R17.
Your Current Tire Size
31.6"
Overall Diameter
186mm
Sidewall Height
99.3"
Circumference
638
Revs per Mile
Tire Height
Stay stock height, or go taller for a more aggressive stance and extra ground clearance.
Alternate Sizes by Wheel Diameter
Pick a wheel size — great for upsizing to bigger wheels without changing your overall tire height.
Staying within ±3% of your original overall diameter keeps your speedometer, odometer, ABS, and traction control working accurately. Sizes within ±2% of the selected height are marked Best Match. Flotation sizes (like 33x12.50R17) use their nominal diameter. The % difference column always compares against your current tire. Always verify fender and suspension clearance before changing width, height, or wheel diameter — taller setups may require trimming, a leveling kit, or a lift.
Compare Two Tire Sizes
Speedometer Calculator
See how changing tire size affects your speedometer reading
Original Tire Size
265/65R18
31.56" diameter
New Tire Size
285/70R18
33.71" diameter
Diameter Difference
+2.15"
Speedometer Error
+6.8%
Your speedometer will read SLOWER than actual speed. At a displayed 60 mph, you're actually going 64.1 mph.
| Speedometer Shows | Actual Speed | Difference |
|---|---|---|
| 30 mph | 32.0 mph | +2.0 mph |
| 45 mph | 48.1 mph | +3.1 mph |
| 60 mph | 64.1 mph | +4.1 mph |
| 70 mph | 74.8 mph | +4.8 mph |
| 80 mph | 85.4 mph | +5.4 mph |
Revolutions per Mile
Original: 639
New: 598
Circumference
Original: 99.2"
New: 105.9"
How Tire Size Math Works
Overall Diameter
Overall diameter = wheel diameter + 2 × sidewall height. Sidewall height is the tire width × aspect ratio. For a 265/70R17: sidewall = 265mm × 70% = 185.5mm (7.3"), so overall diameter = 17" + 2 × 7.3" = 31.6".
The 3% Rule
Any replacement size within ±3% of your original overall diameter is generally safe — your speedometer stays accurate and your drivetrain components see the gearing they were designed for. We flag sizes within ±2% as Best Match.
Plus Sizing
Going from 17" to 20" wheels? Pick the 20" chip above and you'll see every tire size that keeps your overall diameter within 3% — shorter sidewall, bigger wheel, same speedometer reading.
Revolutions per Mile
A tire travels its circumference each revolution. Fewer revs per mile means a taller tire — which also means lower effective gearing and a speedometer that reads under your true speed.
Tire Size FAQs
How do I read a tire size like 265/70R17?
The first number (265) is the tire width in millimeters. The second (70) is the aspect ratio — the sidewall height as a percentage of the width. R means radial construction, and 17 is the wheel diameter in inches.
How much can my tire diameter change safely?
The industry rule of thumb is to stay within 3% of your original overall diameter. Within that range, your speedometer, odometer, ABS, and traction control keep working accurately. Within 2% is ideal.
What happens if I put bigger wheels on my vehicle?
When you increase wheel diameter, you need a tire with a shorter sidewall to keep the same overall diameter — this is called plus sizing. Our calculator shows exactly which tire sizes work for each wheel diameter.
Will a different tire size affect my speedometer?
Yes. A larger overall diameter makes your speedometer read slower than your true speed, and a smaller diameter makes it read faster. The calculator shows your actual speed at an indicated 60 mph for every alternate size.
Found Your Size?
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