Tire Size Calculator

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tire size calculator
Single Tire DimensionsCompare Two Tire Sizes
New Tire Specification:
Analysis Results:
Enter tire specifications and click Calculate to see the results.
function toggleInputs(){var mode=document.getElementById('calc_type').value;var t2div=document.getElementById('tire2_divider');var t2cont=document.getElementById('tire2_container');if(mode==='single'){t2div.style.display='none';t2cont.style.display='none';document.getElementById('label_w1′).innerText='Width (mm):';}else{t2div.style.display='block';t2cont.style.display='table';document.getElementById('label_w1′).innerText='Current Width (mm):';}}function calculateTire(){var w1=parseFloat(document.getElementById('width1').value);var a1=parseFloat(document.getElementById('aspect1').value);var r1=parseFloat(document.getElementById('wheel1').value);var mode=document.getElementById('calc_type').value;if(isNaN(w1)||isNaN(a1)||isNaN(r1)){alert('Please enter valid tire specs');return;}var s1=(w1*(a1/100))/25.4;var d1=(s1*2)+r1;var c1=d1*Math.PI;var rev1=63360/c1;var out=";if(mode==='single'){out='';out+='';out+='';out+='';out+='';out+='
Sidewall Height:'+s1.toFixed(2)+' in
Total Diameter:'+d1.toFixed(2)+' in
Circumference:'+c1.toFixed(2)+' in
Revs per Mile:'+rev1.toFixed(2)+'
';}else{var w2=parseFloat(document.getElementById('width2').value);var a2=parseFloat(document.getElementById('aspect2').value);var r2=parseFloat(document.getElementById('wheel2').value);if(isNaN(w2)||isNaN(a2)||isNaN(r2)){alert('Please enter specs for both tires');return;}var s2=(w2*(a2/100))/25.4;var d2=(s2*2)+r2;var c2=d2*Math.PI;var rev2=63360/c2;var diff=((d2-d1)/d1)*100;var speedo=60*(d2/d1);out='';out+='';out+='';out+='';out+='';out+='';out+='';out+='
MetricTire 1Tire 2Difference
Diameter'+d1.toFixed(2)+'"'+d2.toFixed(2)+'"'+(d2-d1).toFixed(2)+'" ('+diff.toFixed(1)+'%)
Width'+w1+'mm'+w2+'mm'+(w2-w1)+'mm
Sidewall'+s1.toFixed(2)+'"'+s2.toFixed(2)+'"'+(s2-s1).toFixed(2)+'"
Circum.'+c1.toFixed(2)+'"'+c2.toFixed(2)+'"'+(c2-c1).toFixed(2)+'"
Revs/Mile'+rev1.toFixed(0)+''+rev2.toFixed(0)+''+(rev2-rev1).toFixed(0)+'
';out+='
Speedometer Note: When your speedometer reads 60 mph, you are actually traveling '+speedo.toFixed(1)+' mph.
';}document.getElementById('answer').innerHTML=out;}toggleInputs();

Using the Tire Size Calculator

Choosing the right rubber for your vehicle is more than just an aesthetic choice; it's a matter of safety, performance, and mechanical integrity. Our tire size calculator is designed to help you compare your stock tire specifications with potential replacements to understand how changes in width, profile, and wheel diameter affect your vehicle.

Whether you are "plus-sizing" your wheels for better handling or looking for more sidewall for off-road comfort, this tool provides the exact dimensions you need to ensure a proper fit.

Tire Width (mm)
This is the nominal width of the tire in millimeters from sidewall to sidewall (e.g., 225).
Aspect Ratio (%)
The height of the sidewall expressed as a percentage of the tire width (e.g., 45 means the sidewall is 45% of 225mm).
Wheel Diameter (in)
The diameter of the metal rim in inches (e.g., 17 or 18).

How the Math Works

Tire sizing uses a mix of metric and imperial units. To calculate the total diameter, we must convert the sidewall height from millimeters to inches. The standard conversion factor is 25.4 millimeters per inch.

Total Diameter = ((Width × Aspect Ratio / 100) / 25.4 × 2) + Wheel Diameter

  • Sidewall Height: Width multiplied by Aspect Ratio.
  • Total Diameter: The sidewall height occurs twice (top and bottom) plus the rim diameter.
  • Circumference: Total Diameter multiplied by Pi (π).
  • Revs Per Mile: 63,360 (inches in a mile) divided by the Circumference.

Speedometer Error Explained

Your vehicle's speedometer is calibrated to the factory tire diameter. It counts the rotations of the output shaft or wheel hubs. If you install a larger tire, the vehicle covers more ground per revolution, meaning you are moving faster than the speedometer indicates.

Example Comparison:

  1. Stock Tire: 215/65R15 (Diameter: 26.00″)
  2. New Tire: 235/75R15 (Diameter: 28.88″)
  3. Difference: 11.1% Increase
  4. Result: If your speedometer says 60 MPH, you are actually doing 66.7 MPH. This could lead to speeding tickets or unexpected transmission shift points.

Common Tire Sizing Questions

Is it safe to change tire sizes?

Generally, automotive experts recommend staying within 3% of the original diameter. Going beyond this can put extra stress on brakes, suspension components, and may cause the tire to rub against the wheel well during sharp turns.

What is "Plus Sizing"?

Plus sizing is increasing the wheel diameter (rim size) while decreasing the aspect ratio (sidewall height) to keep the overall tire diameter the same. This improves steering response and gives the car a modern look without affecting the speedometer.

How does width affect MPG?

Wider tires have a larger contact patch, which increases rolling resistance and aerodynamic drag. While they offer more grip, they typically result in a slight decrease in fuel economy compared to narrower tires.

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