Actual Speed Calculator

This calculator works by multiplying the speed on the dashboard by the ratio of the new tire diameter to the old tire diameter. It returns your true road speed after a tire change.

1. Enter the original size, for example 265/70R17 (31.61 in).
2. Enter the new size, for example 285/75R17 (33.83 in).
3. Enter the indicated speed, for example 65 mph.
4. Read the actual speed: 69.6 mph (112.0 km/h).

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

Worked example: at an indicated 50 mph (80 km/h), the true speed is 50 × 1.0704 = 53.5 mph (86.1 km/h).

The ratio is constant at every speed. A 7.04% tire change adds 7.04% to every reading.

The conversion table shows actual speed for 13 indicated speeds after a 265/70R17 to 285/75R17 change. Indicated speeds run from 20 mph (32 km/h) to 80 mph (129 km/h).

The gap grows with speed. At 25 mph the true speed is 1.8 mph higher; at 80 mph it is 5.6 mph higher.

The Actual Speed Calculator is a tool that converts the speed on your dashboard into true road speed after fitting a different tire size. It is the tool to use while driving a vehicle that has not been recalibrated.

The calculator focuses on one direction: from what the dial says to what the road speed is. It suits drivers who check the table before a trip through speed cameras or school zones.

This calculator has 4 uses: school-zone checks, highway cruising, speed-camera zones and towing limits.

• Check school zones: Know true speed at an indicated 20 or 25 mph.
• Cruise on highways: Set cruise below the limit by the right margin.
• Pass speed cameras: Avoid fines in camera zones.
• Respect towing limits: Stay under trailer tire speed ratings.