Tire Size Calculator
Compare tire sizes, calculate dimensions, and see speedometer/odometer differences.
Current Tire Size
Example: 225/55R17
New Tire Size
Example: 235/50R18
Tire Dimensions
| Current | New | Diff | |
|---|---|---|---|
| Diameter | 26.7" | 27.3" | +0.51" |
| Width | 8.9" | 9.3" | +0.39" |
| Sidewall | 4.87" | 4.63" | -0.25" |
| Revs/Mile | 754 | 740 | -14 |
Compatible Size
+1.9%
Diameter difference (recommend ±3% max)
Speedometer & Odometer Effects
Speedometer ReadingReads slow by 1.9%
At 60 mph indicated, actual speed: 61.1 mph
Odometer ReadingRecords fewer miles by 1.9%
Speed Conversion Table
| Indicated | Actual | Error |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 mph |
| 45 mph | 45.9 mph | +0.9 mph |
| 60 mph | 61.1 mph | +1.1 mph |
| 70 mph | 71.3 mph | +1.3 mph |
| 80 mph | 81.5 mph | +1.5 mph |
Compatible Sizes
205/55R16
24.9"-7.0%
215/55R17OK
26.3"-1.6%
225/55R17OK
26.7"+0.0%
225/45R18OK
26.0"-2.9%
235/50R18OK
27.3"+1.9%
245/45R19
27.7"+3.5%
255/40R19OK
27.0"+1.1%
265/35R20OK
27.3"+2.1%
Understanding Tire Size Notation
Tire sizes use a standardized format that specifies width, aspect ratio, construction type, and wheel diameter. Example: P225/45R17 91V
| Component | Position | Meaning | Example Value |
|---|---|---|---|
| Service Type | Prefix (optional) | P=Passenger, LT=Light Truck, T=Temporary | P |
| Width | First 3 digits | Tire width in millimeters | 225 mm |
| Aspect Ratio | After / | Sidewall height as % of width | 45% of 225mm = 101mm |
| Construction | Letter | R=Radial, D=Diagonal/Bias, B=Belted | R (Radial) |
| Rim Diameter | After R | Wheel diameter in inches | 17 inches |
| Load Index | After diameter | Maximum load capacity code | 91 = 1,356 lbs |
| Speed Rating | Last letter | Maximum safe speed | V = 149 mph |
Tire Dimension Calculations
Calculate tire dimensions from the size code:
| Calculation | Formula | Example (225/45R17) | Result |
|---|---|---|---|
| Sidewall Height | Width × (Aspect ÷ 100) | 225 × 0.45 | 101.25 mm |
| Sidewall (inches) | Sidewall mm ÷ 25.4 | 101.25 ÷ 25.4 | 3.99 inches |
| Overall Diameter | (Sidewall × 2) + Rim | (3.99 × 2) + 17 | 24.98 inches |
| Circumference | π × Overall Diameter | 3.14159 × 24.98 | 78.47 inches |
| Revolutions/Mile | 63,360 ÷ Circumference | 63,360 ÷ 78.47 | 807 rev/mi |
| Width (inches) | Width mm ÷ 25.4 | 225 ÷ 25.4 | 8.86 inches |
Tire Size Formulas
Sidewall Height = Width × (Aspect/100)
Overall Diameter = (Sidewall × 2) + Wheel Diameter
Circumference = π × Overall Diameter
Revs/Mile = 63360 / Circumference (inches)
Where:
- Aspect= Aspect ratio percentage
- Width= Tire width in mm
Common Tire Size Dimensions
Quick reference for popular tire sizes:
| Tire Size | Diameter (in) | Width (in) | Circumference (in) | Revs/Mile |
|---|---|---|---|---|
| 205/55R16 | 24.9 | 8.1 | 78.2 | 810 |
| 215/55R17 | 26.3 | 8.5 | 82.6 | 767 |
| 225/45R17 | 25.0 | 8.9 | 78.5 | 807 |
| 225/60R17 | 27.6 | 8.9 | 86.7 | 731 |
| 235/55R18 | 28.2 | 9.3 | 88.5 | 716 |
| 245/45R18 | 26.7 | 9.6 | 83.8 | 756 |
| 265/70R17 | 31.6 | 10.4 | 99.3 | 638 |
| 275/55R20 | 31.9 | 10.8 | 100.2 | 632 |
Tire Speed Ratings
Speed ratings indicate the maximum speed a tire can safely maintain:
| Rating | Max Speed (mph) | Max Speed (km/h) | Typical Usage |
|---|---|---|---|
| Q | 99 | 160 | Winter tires |
| R | 106 | 170 | Heavy-duty light trucks |
| S | 112 | 180 | Family sedans, vans |
| T | 118 | 190 | Family sedans, minivans |
| U | 124 | 200 | Sedans, coupes |
| H | 130 | 210 | Sport sedans, coupes |
| V | 149 | 240 | Sports cars, performance |
| W | 168 | 270 | Exotic sports cars |
| Y | 186 | 300 | High-performance exotics |
| Z (ZR) | 149+ | 240+ | High-performance (varies) |
Tire Load Index Reference
Load index indicates the maximum weight each tire can support:
| Load Index | Max Load (lbs) | Max Load (kg) | Common Vehicle Types |
|---|---|---|---|
| 85 | 1,135 | 515 | Compact cars |
| 89 | 1,279 | 580 | Midsize sedans |
| 91 | 1,356 | 615 | Sedans, small SUVs |
| 94 | 1,477 | 670 | Large sedans, SUVs |
| 97 | 1,609 | 730 | SUVs, crossovers |
| 100 | 1,764 | 800 | Large SUVs, trucks |
| 104 | 1,984 | 900 | Light trucks |
| 110 | 2,337 | 1,060 | Heavy-duty trucks |
| 116 | 2,756 | 1,250 | Commercial vehicles |
| 121 | 3,197 | 1,450 | Heavy trucks, trailers |
Total vehicle load capacity = 4 × single tire load index rating (minus vehicle weight for available cargo capacity)
Plus Sizing: Upgrading Wheel and Tire Size
Plus sizing involves larger wheels with lower-profile tires while maintaining overall diameter:
| Original | Plus 1 | Plus 2 | Overall Diameter | Change |
|---|---|---|---|---|
| 205/55R16 | 215/45R17 | 225/40R18 | 24.9" | Stock |
| 215/60R16 | 225/50R17 | 235/45R18 | 26.2" | Stock |
| 225/45R17 | 235/40R18 | 245/35R19 | 25.0" | Stock |
| 235/55R17 | 245/50R18 | 255/45R19 | 27.2" | Stock |
| Plus Sizing | Advantages | Disadvantages |
|---|---|---|
| Benefits | Better handling, improved grip, sportier appearance | Harsher ride, more expensive, more road noise |
| Considerations | Larger contact patch, quicker steering response | Increased hydroplaning risk, shorter tread life |
| Keep Same | Overall diameter within 3% of original | Affects speedometer, ABS, traction control |
Effects of Changing Tire Size
How different tire sizes affect your vehicle:
| Change | Speedometer Effect | MPG Effect | Other Effects |
|---|---|---|---|
| 3% larger diameter | Shows 3% slower than actual | +1% to +2% | Slightly lower RPM, raised ride height |
| 3% smaller diameter | Shows 3% faster than actual | -1% to -2% | Slightly higher RPM, lowered ride height |
| Wider tire | Minimal effect | -1% to -3% | Better grip, more rolling resistance |
| Lower aspect ratio | Depends on diameter change | Varies | Stiffer ride, sharper handling |
| Higher aspect ratio | Depends on diameter change | Varies | Softer ride, more sidewall flex |
| Original Size | New Size | Diameter Diff | Speedo at 70 mph |
|---|---|---|---|
| 225/45R17 (25.0") | 225/50R17 (25.9") | +3.6% | Actually 72.5 mph |
| 225/45R17 (25.0") | 215/45R17 (24.6") | -1.6% | Actually 68.9 mph |
| 225/45R17 (25.0") | 235/45R17 (25.3") | +1.2% | Actually 70.8 mph |
Worked Examples
Calculate Tire Dimensions
Problem:
What are the dimensions of a 245/40R18 tire?
Solution Steps:
- 1Width: 245 mm = 9.65 inches
- 2Aspect ratio: 40% of width
- 3Sidewall height: 245 × 0.40 = 98 mm = 3.86 inches
- 4Overall diameter: (3.86 × 2) + 18 = 25.72 inches
- 5Circumference: π × 25.72 = 80.79 inches
- 6Revolutions per mile: 63,360 ÷ 80.79 = 784
Result:
245/40R18: 25.7" diameter, 9.65" width, 784 revs/mile
Compare Tire Sizes for Upgrade
Problem:
Can you replace 215/55R17 tires with 225/50R17? What's the difference?
Solution Steps:
- 1Original (215/55R17): Sidewall = 215 × 0.55 = 118.25 mm
- 2Original diameter: (118.25 × 2 / 25.4) + 17 = 26.31 inches
- 3New (225/50R17): Sidewall = 225 × 0.50 = 112.5 mm
- 4New diameter: (112.5 × 2 / 25.4) + 17 = 25.87 inches
- 5Diameter difference: (25.87 - 26.31) / 26.31 = -1.67%
Result:
The 225/50R17 is 1.67% smaller in diameter—within acceptable 3% range but speedometer will read slightly fast
Calculate Speedometer Error
Problem:
Your speedometer shows 60 mph with original 225/45R17 tires. You switched to 235/50R17. What's your actual speed?
Solution Steps:
- 1Original 225/45R17: Diameter = (225 × 0.45 × 2 / 25.4) + 17 = 24.97"
- 2New 235/50R17: Diameter = (235 × 0.50 × 2 / 25.4) + 17 = 26.26"
- 3Size ratio: 26.26 / 24.97 = 1.052 (5.2% larger)
- 4Actual speed: 60 × 1.052 = 63.1 mph
- 5Speedometer reads 60 but you're actually going 63.1 mph
Result:
With the larger tires, you're actually traveling 63.1 mph when the speedometer shows 60 mph
Tips & Best Practices
- ✓Stay within 3% of original tire diameter to maintain accurate speedometer, ABS, and traction control function
- ✓Use a tire size calculator to verify diameter, circumference, and speedometer impact before purchasing new sizes
- ✓For AWD vehicles, all four tires must be the same size and within 2/32" tread depth of each other
- ✓Lower aspect ratio tires look sportier and handle better but provide a harsher ride and are more prone to pothole damage
- ✓Always match or exceed the original tire's load index and speed rating for safety
- ✓Check the manufacture date (DOT code) when buying tires—avoid tires more than 2 years old
- ✓When plus-sizing, compensate for larger wheels with lower-profile tires to maintain the same overall diameter
Frequently Asked Questions
The general rule is to stay within 3% of the original tire's overall diameter. Larger changes affect speedometer accuracy, ABS/traction control systems, gear ratios, and fuel economy. Changes larger than 3% may trigger warning lights or cause problems with electronic stability systems. Also ensure the tires fit within the wheel wells without rubbing during turns or over bumps. Check your vehicle's manual for approved alternative sizes.
AWD systems distribute power between axles based on wheel speed differences. If tires have different diameters (even from wear differences), the system constantly tries to compensate, causing excessive wear on differentials and transfer cases. For AWD vehicles: use matching tires on all four corners, replace tires in sets of 4, and if one tire is damaged, consider shaving a new tire to match the wear of the others. Diameter differences should be less than 2/32" in tread depth.
The aspect ratio (sidewall height as percentage of width) significantly affects ride quality. Lower aspect ratios (35-45) mean shorter, stiffer sidewalls: sharper handling and steering response but harsher ride, more road noise, and increased susceptibility to pothole damage. Higher aspect ratios (55-70) provide more cushioning: smoother ride and better impact absorption but more body roll and less precise handling. Performance cars use low profiles; comfort-oriented vehicles use higher profiles.
Staggered setups (wider rear tires) are common on rear-wheel-drive sports cars and improve traction and appearance. However: 1) Never use different sizes on the same axle, 2) Staggered tires can't be rotated front-to-rear, 3) AWD vehicles generally shouldn't use staggered fitments, 4) Ensure the overall diameters are similar between front and rear (within 1-2%). Your vehicle's specifications will indicate if staggered fitment is approved.
Using tires with a lower load index than the original specification is dangerous. The tires may fail under normal driving conditions, especially when carrying passengers or cargo. This is particularly critical for trucks, SUVs, and vehicles that tow. Always use tires with a load index equal to or greater than the original equipment specification. The load index affects the tire's internal construction, not just a rating—underrated tires will flex excessively, overheat, and can blow out.
The DOT code on the sidewall ends with a 4-digit date code: first two digits are the week, last two are the year. For example, '2419' means the tire was made in week 24 (June) of 2019. Tires degrade over time even when unused. Most manufacturers recommend replacing tires after 6 years regardless of tread depth, and tires over 10 years old should never be used. When buying new tires, check the date code—avoid purchasing tires more than 1-2 years old.
Sources & References
Last updated: 2026-01-22