Kelstad Wheels

Kelstad Wheels computes tyre geometry from three numbers: section width, aspect ratio, rim diameter.

Four tools.

1. Compare

Enter two sizes, for example 225/45R17 against 235/40R18. Each side returns sidewall height in mm, overall diameter in mm, circumference in mm, and revolutions per kilometre. The delta panel reports the diameter difference in mm and in percent, then states a verdict: within tolerance at 3 percent or below, out of tolerance above it. A silhouette draws both tyres side by side, scaled to the computed diameters.

2. Speedo

Set the stock size and the alternative size. The table lists indicated speed against true speed at 30, 50, 70, 90, 110 and 130 km/h, with the error percentage on each row. True speed = indicated × alternative diameter ÷ stock diameter. The error is constant across the range. A summary line gives the figure and states whether the speedometer over-reads or under-reads.

3. Index

Load index 70 to 110, resolved to kilograms per tyre. Speed ratings N, P, Q, R, S, T, H, V, W and Y, resolved to km/h. One filter field narrows both tables at once. A reference card breaks down a sidewall marking, digit by digit.

4. Garage

Saved setups hold a vehicle name, a stock size, an alternative size and a note. Every row carries its own diameter delta and tolerance verdict. The header strip counts the setups and how many of them fall outside tolerance. Tap a setup to load it into Compare. Long-press to edit it.

Specifications

Tolerance limit: 3 percent of stock diameter.
Speed steps: 30, 50, 70, 90, 110, 130 km/h.
Load index range: 70 to 110.
Speed ratings: 10 letters, 140 to 300 km/h.
Every figure is computed on the device.
Three example setups are present on first run.
Portrait orientation. Dark theme. Silent.

Works without internet