Indicated Speed Calculator

This calculator works by dividing a true road speed by the ratio of the new diameter to the old diameter. It tells you what the speedometer shows when you travel at a chosen real speed.

1. Enter the factory size, for example 245/75R16 (30.47 in).
2. Enter the new size, for example 285/75R16 (32.83 in).
3. Enter the true speed you want, for example 100 km/h (62.1 mph).
4. Read the indicated speed: 92.8 km/h (57.7 mph).

The formula is Indicated Speed = Actual Speed × (Old Diameter ÷ New Diameter).

Worked example: to drive at a true 120 km/h (74.6 mph), the speedometer must show 120 × 0.9281 = 111.4 km/h (69.2 mph).

The conversion table shows the dashboard reading at 11 true speeds from 30 km/h to 130 km/h after a 245/75R16 to 285/75R16 change. The tire change is +7.75%.

Read the target speed in the left column. The indicated km/h column is the number to watch on the dial.

The Indicated Speed Calculator is a tool that finds the speedometer reading that matches a chosen true road speed. It is the reverse of the actual speed calculator.

Drivers use it to set cruise control after an upsize. To travel at the legal limit, you aim for the indicated figure in the table.

This calculator has 4 uses: cruise targets, limit tables, GPS cross-checks and trip planning.

• Set cruise targets: Find the dial speed for a true legal speed.
• Print limit tables: Make a dash card for common limits.
• Cross-check GPS: Predict what the dial shows at a GPS speed.
• Plan trips: Estimate travel times using true speed.