Accuracy and limits
Datum gives geometric estimates from the numbers you enter. It does not say that a tire or wheel will fit your vehicle, and it cannot.
Not a fitment guarantee. Tires and wheels are safety-critical parts. Before you buy or fit any, confirm the size, load rating, speed rating and clearances with your vehicle’s maker, the tire and wheel makers and a qualified installer. Datum helps you ask better questions. It does not replace their answers.
What the results are
Every figure comes from a short formula applied to the sizes and measurements you type. The method page lists each formula. The results describe the geometry of two setups and how they differ. They are estimates of that geometry, not measurements of your vehicle.
Where the numbers can differ from the real world
- Tire size. Sizes are nominal. A real tire’s height and width depend on the model, the tread depth, wear, air pressure, load and even speed. Enter the maker’s figures under “Exact figures from the tire maker” when you have them.
- Width on the wheel. A tire’s published width is measured on a specific wheel. Datum estimates how the width changes on yours, using a rule of thumb that can be off by 5 mm or more. It is labelled as an estimate everywhere it appears.
- Wheel dimensions. Overall wheel width is approximated as the stamped width plus one inch. Real wheels differ a little, and offsets and backspacing vary within manufacturing tolerances.
- Speedometer. The speedometer figures assume your speedometer was accurate on the original tires and that it reads from the wheels. Many vehicles are not perfectly accurate to begin with. A GPS speed reading is a good check.
- Your measurements. If you enter your own clearance measurements, the comparison is only as good as those measurements and covers only the points you measured.
What the tool cannot see
- The shape of your fender, suspension travel, steering lock, camber, body roll and ride height under load.
- The inside of the wheel: spoke shape, brake caliper and rotor clearance, hub and centre bore, bolt pattern and nut seat.
- Whether a tire is approved for a wheel width, and whether the tire and wheel are rated for your vehicle’s weight and top speed.
- How your vehicle’s speedometer, transmission, ABS, traction control, stability control and tire-pressure monitoring respond to a different size.
- Local rules about how far tires may stick out, fender coverage, speedometer accuracy, inspection and insurance.
What Datum will never say
Datum will never tell you that a tire or wheel fits, clears, is safe or is approved. When you enter your own measurements it compares each setup with them and reports the difference, and it reminds you what that comparison does not include.
No professional advice, no liability
The site is information only. It is not professional advice, and the operator accepts no liability for decisions made using it, to the extent the law allows. The terms of use set this out in full.
Found a mistake?
If a number, a formula or a guide looks wrong, please tell us at supportdatum at gmail dot com. Include the comparison link if you can, since it holds the exact numbers.