Tire Calculator - Compare Tire Sizes (Articles)

Tire calculator to compare car tires, tire diameter and rolling circumference are calculated, as well as the change in actual speed and vehicle height/ground clearance.

<script>function perRound(num, precision) { var precision = 1; precision = parseInt(precision); var result1 = num * Math.pow(10, precision); var result2 = Math.round(result1); var result3 = result2 / Math.pow(10, precision); return zerosPad(result3, precision); } function zerosPad(rndVal, decPlaces) { var valStrg = rndVal.toString(); var decLoc = valStrg.indexOf("."); if (decLoc == -1) { decPartLen = 0; valStrg += decPlaces > 0 ? "." : ""; } else { decPartLen = valStrg.length - decLoc - 1; } var totalPad = decPlaces - decPartLen; if (totalPad > 0) { for (var cntrVal = 1; cntrVal <= totalPad; cntrVal++) valStrg += "0"; } return valStrg; } function clear_field(field) { if (field.value==field.defaultValue) { field.value='' } }function nofocus() { document.TireCalc.InUnit.focus() } var Pi = 3.14159; function CalcSize() { var width = parseFloat(document.all.twIn.value);var width2 = parseFloat(document.all.twIn2.value);var aspectRatio = parseFloat(document.all.arIn.value);var aspectRatio2 = parseFloat(document.all.arIn2.value);var rimDia = parseFloat(document.all.rdIn.value) * 25.4;var tireH = width * aspectRatio / 100;var rimDia2 = parseFloat(document.all.rdIn2.value) * 25.4;var tireH = width * aspectRatio / 100;var tireH2 = width2 * aspectRatio2 / 100;var dia = (2 * tireH) + rimDia;var dia2 = (2 * tireH2) + rimDia2;var diaRounded = perRound(dia);var diaRounded2 = perRound(dia2);var tdrdiff = perRound(((dia2/dia) - 1) * 100);var tdrdiffh = perRound((dia2 - dia) / 2);document.TireCalc.tdr.value = diaRounded;var circum = perRound(diaRounded * Pi); document.TireCalc.tcr.value = circum;document.TireCalc.tdr2.value = diaRounded2;var circum2 = perRound(diaRounded2 * Pi); document.TireCalc.tcr2.value = circum2;document.TireCalc.tdrdiff.value = tdrdiff;document.TireCalc.tdrdiffh.value = tdrdiffh;</script>
<form>Enter Tire DataReference TireComparison TireTire Width mmSidewall Ratio %Wheels

mm	Tire Width<input>		<input>
	<input>Sidewall Ratio		%<input>
size	<input> inches	Wheel size	<input>
<input><input>
Result
Wheel diameter	<input> mm	Wheel diameter	<input>
	<input>	Wheel circumference mmWheel circumang	<input> mm
Change in speed/Change in wheel circumference			<input> %
Change in ground clearance/Change in vehicle height			<input> mm

</form>

Example: Tire size 205/60 R16 H

"205" is the tire width in millimeters, "60" is the tire height to width ratio in percent, and "16" is the wheel size in inches. "H" is the speed index, which indicates that the tire is approved for use up to 210 km/h.

Speed Indices:

L: 120km/h
• M: 130km/h
N: 140km/h
• P: 150km/h
Q: 160km/h
• R: 170km/h
S: 180km/h
• T: 190km/h
U: 200km/h
• H: 210km/h
V: 240km/h
• VR: >240km/h
W: 270km/h
• ZR: >240km/h
Y: 300km/h

For "ZR", the speed index Y is often also specified, for example, 225/45 ZR17 94Y.

The load index, also found on the tire sidewall, indicates the tire's load capacity. One can read the load-bearing capacity from a larger table or simply calculate it: T(kg) = 45 * 10 EXP (Load-bearing index / 80)
For example, with a load-bearing index of 94, the load-bearing capacity in kg is calculated as follows: 45 * 10 EXP (94 / 80) = 45 * 14.96 = 673