Tyre Size Calculator
Compare original and proposed tyre dimensions with a live, proportional visual.
Compare tyre sizes
Enter the size moulded on each tyre. Each tyre can use Metric, Inches or a verified Other catalogue size independently, and the results update automatically.
Please enter a valid value in every field.
Overall diameter change
The proposed tyre is larger in overall diameter.
Tyre Size Comparison Visualizer
Tyre width difference: +50.0 mm (+21.28%)
Speedometer Error
Assuming the speedometer is calibrated to the original tyre size.
| Indicated | 10 | 30 | 50 | 70 | 90 | 110 | 130 | 150 km/h |
|---|---|---|---|---|---|---|---|---|
| Actual | 10.3 | 30.9 | 51.5 | 72.2 | 92.8 | 113.4 | 134.0 | 154.6 km/h |
Calculation Results
| Measurement | Original Size 235/85R16 | New Size 285/70R17 |
|---|---|---|
| Overall Diameter (mm) | 806 | 831 |
| Overall Diameter (in) | 31.73 | 32.71 |
| Sidewall Height (mm) | 200 | 200 |
| Sidewall Height (in) | 7.86 | 7.85 |
| Tread Width (mm) | 235 | 285 |
| Tread Width (in) | 9.25 | 11.22 |
| Difference | 0 mm (0.00%) | +25 mm (+3.10%) |
Summary millimetre values are rounded to the nearest whole millimetre. The additional results below retain decimal precision.
Additional Results
| Measurement | Original | Proposed | Difference |
|---|---|---|---|
| Overall diameter | 805.9 mm / 31.73 in | 830.8 mm / 32.71 in | +24.9 mm (+3.09%) |
| Sidewall height | 199.8 mm / 7.86 in | 199.5 mm / 7.85 in | -0.3 mm (-0.13%) |
| Section width | 235.0 mm / 9.25 in | 285.0 mm / 11.22 in | +50.0 mm (+21.28%) |
| Rim diameter | 406.4 mm / 16 in | 431.8 mm / 17 in | +25.4 mm (+6.25%) |
| Circumference | 2531.8 mm | 2610.0 mm | +78.2 mm (+3.09%) |
| Revolutions per kilometre | 395.0 | 383.1 | -11.8 (-3.00%) |
| Ride-height change | Half the overall-diameter change | +12.5 mm | |
| Actual speed at indicated 100 km/h | Assuming the speedometer is calibrated to the original tyre | 103.1 km/h (+3.1 km/h) | |
Tyre Cold Inflation Calculator
Estimate cold inflation pressure from the laden axle-group load, tyre arrangement, and the tyre's marked maximum load and pressure.
PSI–kPa converter
1 psi = 6.89476 kPa • 1 kPa = 0.145038 psi
kg–lb (pounds) converter
1 kg = 2.20462 lb • 1 lb = 0.453592 kg
Steps to use and formulas
- Enter the maximum laden load for each axle group.
- Enter the number of tyres per axle and axles in that axle group.
- Enter the maximum load and cold pressure marked for each tyre.
- Confirm the load per tyre does not exceed the tyre's maximum load, then use the estimated pressure as a starting point only.
Total tyres = tyres per axle × axles in axle group.
Load per tyre = axle-group load ÷ total tyres.
Estimated cold pressure per tyre = tyre maximum pressure × (load per tyre ÷ tyre maximum load).
The front result retains one decimal place to match the supplied workbook. The rear result is rounded up to the next whole PSI, matching that workbook.
Important: This proportional estimate does not replace the vehicle tyre placard, tyre-manufacturer load/inflation tables, or engineering requirements. Confirm the tyre construction, axle loading, operating conditions and applicable standards before use.
Offset and Backspacing
Wheel offset is the distance in millimetres between the wheel centreline and the hub-mounting face. Positive offset generally positions more of the wheel inward; negative offset generally moves it outward. Zero offset places the mounting face on the centreline.
Backspacing is measured from the wheel's rear outer edge to the hub-mounting face. More backspacing places more wheel inside the wheel arch; less backspacing moves it outward.
Marked wheel width is the bead-seat width, not overall outside width. This calculator uses the common approximation of nominal width plus 1 inch. Measure the actual overall rim width when an exact result is required.
Offset and Backspacing Calculator
Backspacing = overall width ÷ 2 + offset ÷ 25.4 • Offset = (backspacing − overall width ÷ 2) × 25.4
Important: Wheel fitment must also account for tyre section width, approved rim width, brake and suspension clearance, steering movement, body clearance, load rating and applicable track requirements.
Wheel Track and Position Calculator
Compare the original and proposed rim widths and offsets. The calculated track assumes unchanged hubs and measures between wheel centrelines.
Enter valid tyre section widths, rim widths, offsets and original track.
Rim and tyre edge measurements
Across the axle, edge to edge. Tyre width is nominal section width; the tyre is not drawn in the position view.
- Rim inside to inside
- —
- Rim outside to outside
- —
- Tyre inside to inside
- —
- Tyre outside to outside
- —
- Rim inside to inside
- —
- Rim outside to outside
- —
- Tyre inside to inside
- —
- Tyre outside to outside
- —
Track change = 2 × (original offset − new offset). Rim width does not change wheel-centre track, but it changes inner clearance and outward projection.
Important: This is a geometric wheel calculation, not a compliance determination. Confirm the applicable wheel-track limits and measure the vehicle before certification.
Tyre Load Index Table
View load index table
Load Index Table
Example:
Tyre 295/80R22.5 G358 152/148 L
- 152 – is the load index for single axle that corresponds to a maximum load of 3,550 kg, an air pressure of 125 Lbs (according to air pressure table), and to a maximum speed of L=120 km/h.
- 148 – is the load index for a tandem axle that corresponds to a maximum load of 3,150 kg, an air pressure of 125 Lbs (according to air pressure table), and to a maximum speed of L=120 km/h.
| LI | kg | LI | kg | LI | kg | LI | kg | LI | kg | LI | kg | LI | kg | LI | kg | LI | kg | LI | kg |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 80 | 450 | 100 | 800 | 120 | 1,400 | 140 | 2,500 | 160 | 4,500 | 180 | 8,000 | 200 | 14,000 | 220 | 25,000 | 240 | 45,000 | 260 | 80,000 |
| 81 | 462 | 101 | 825 | 121 | 1,450 | 141 | 2,575 | 161 | 4,625 | 181 | 8,250 | 201 | 14,500 | 221 | 25,750 | 241 | 46,250 | 261 | 82,500 |
| 82 | 475 | 102 | 850 | 122 | 1,500 | 142 | 2,650 | 162 | 4,750 | 182 | 8,500 | 202 | 15,000 | 222 | 26,500 | 242 | 47,500 | 262 | 85,000 |
| 83 | 487 | 103 | 875 | 123 | 1,550 | 143 | 2,725 | 163 | 4,875 | 183 | 8,750 | 203 | 15,500 | 223 | 27,250 | 243 | 48,750 | 263 | 87,500 |
| 84 | 500 | 104 | 900 | 124 | 1,600 | 144 | 2,800 | 164 | 5,000 | 184 | 9,000 | 204 | 16,000 | 224 | 28,000 | 244 | 50,000 | 264 | 90,000 |
| 85 | 515 | 105 | 925 | 125 | 1,650 | 145 | 2,900 | 165 | 5,150 | 185 | 9,250 | 205 | 16,500 | 225 | 29,000 | 245 | 51,500 | 265 | 92,500 |
| 86 | 530 | 106 | 950 | 126 | 1,700 | 146 | 3,000 | 166 | 5,300 | 186 | 9,500 | 206 | 17,000 | 226 | 30,000 | 246 | 53,000 | 266 | 95,000 |
| 87 | 545 | 107 | 975 | 127 | 1,750 | 147 | 3,075 | 167 | 5,450 | 187 | 9,750 | 207 | 17,500 | 227 | 30,750 | 247 | 54,500 | 267 | 97,500 |
| 88 | 560 | 108 | 1,000 | 128 | 1,800 | 148 | 3,150 | 168 | 5,600 | 188 | 10,000 | 208 | 18,000 | 228 | 31,500 | 248 | 56,000 | 268 | 100,000 |
| 89 | 580 | 109 | 1,030 | 129 | 1,850 | 149 | 3,250 | 169 | 5,800 | 189 | 10,300 | 209 | 18,500 | 229 | 32,500 | 249 | 58,000 | 269 | 103,000 |
| 90 | 600 | 110 | 1,060 | 130 | 1,900 | 150 | 3,350 | 170 | 6,000 | 190 | 10,600 | 210 | 19,000 | 230 | 33,500 | 250 | 60,000 | 270 | 106,000 |
| 91 | 615 | 111 | 1,090 | 131 | 1,950 | 151 | 3,450 | 171 | 6,150 | 191 | 10,900 | 211 | 19,500 | 231 | 34,500 | 251 | 61,500 | 271 | 109,000 |
| 92 | 630 | 112 | 1,120 | 132 | 2,000 | 152 | 3,550 | 172 | 6,300 | 192 | 11,200 | 212 | 20,000 | 232 | 35,500 | 252 | 63,000 | 272 | 112,000 |
| 93 | 650 | 113 | 1,150 | 133 | 2,060 | 153 | 3,650 | 173 | 6,500 | 193 | 11,500 | 213 | 20,600 | 233 | 36,500 | 253 | 65,000 | 273 | 115,000 |
| 94 | 670 | 114 | 1,180 | 134 | 2,120 | 154 | 3,750 | 174 | 6,700 | 194 | 11,800 | 214 | 21,200 | 234 | 37,500 | 254 | 67,000 | 274 | 118,000 |
| 95 | 690 | 115 | 1,215 | 135 | 2,180 | 155 | 3,875 | 175 | 6,900 | 195 | 12,150 | 215 | 21,800 | 235 | 38,750 | 255 | 69,000 | 275 | 121,500 |
| 96 | 710 | 116 | 1,250 | 136 | 2,240 | 156 | 4,000 | 176 | 7,100 | 196 | 12,500 | 216 | 22,400 | 236 | 40,000 | 256 | 71,000 | 276 | 125,000 |
| 97 | 730 | 117 | 1,285 | 137 | 2,300 | 157 | 4,125 | 177 | 7,300 | 197 | 12,850 | 217 | 23,000 | 237 | 41,250 | 257 | 73,000 | 277 | 128,500 |
| 98 | 750 | 118 | 1,320 | 138 | 2,360 | 158 | 4,250 | 178 | 7,500 | 198 | 13,200 | 218 | 23,600 | 238 | 42,500 | 258 | 75,000 | 278 | 132,000 |
| 99 | 775 | 119 | 1,360 | 139 | 2,430 | 159 | 4,375 | 179 | 7,750 | 199 | 13,600 | 219 | 24,300 | 239 | 43,750 | 259 | 77,500 | 279 | 136,000 |
Scroll horizontally to view the complete table on smaller screens.
Load-index capacities align with the internationally used LI scale, including UNECE Regulation No. 54 commercial-tyre data and the higher specialist range published in ISO 4250-1.