Indicated Speed Calculator
Find what your speedometer will show at a given true road speed after a tire size change. Set cruise control to match posted limits.
Estimate displayed speed at a given road speed.
Indicated Speed Calculator: Tire Size Inputs
Compare two tire sizes here. Use the formulas and reference tables below for this tool's specific calculation.
How Indicated Speed Calculator Works?
The Indicated Speed Calculator works by dividing the true road speed by the ratio of the new tire diameter to the calibrated tire diameter. The tool uses 3 steps.
Enter both sizes
Type the calibrated size and the fitted size.
Enter the target true speed
Type the posted speed limit or the GPS speed, for example 65 mph (105 km/h).
Read the dash value
The calculator returns the speed the speedometer shows when the vehicle moves at the target true speed.
Indicated Speed Calculator Formula
The Indicated Speed Calculator formula is V_indicated = V_actual × (OD_cal ÷ OD_new).
V_i = V_a × OD_cal ÷ OD_new1 ÷ CF = OD_cal ÷ OD_new- V_a
- True road speed
- V_i
- Speed shown on the dash
- CF
- Correction factor OD_new ÷ OD_cal
Worked example
A truck calibrated for 265/70 R17 runs 285/75 R17 tires (CF = 1.0704). To travel at a true 65 mph (104.6 km/h), the driver holds an indicated 60.7 mph (97.7 km/h).
Indicated Speed Calculator Conversion Table
The Indicated Speed Calculator conversion table lists the dash reading at 8 true speeds for a car and a truck with upsized tires.
| True speed | Car: 255/35 R19 on 225/45 R17 cal. | Truck: 285/75 R17 on 265/70 R17 cal. | Truck (km/h) |
|---|---|---|---|
| 30 mph (48 km/h) | 28.8 mph | 28.0 mph | 45.1 km/h |
| 40 mph (64 km/h) | 38.4 mph | 37.4 mph | 60.1 km/h |
| 50 mph (80 km/h) | 48.0 mph | 46.7 mph | 75.2 km/h |
| 55 mph (89 km/h) | 52.8 mph | 51.4 mph | 82.7 km/h |
| 60 mph (97 km/h) | 57.6 mph | 56.1 mph | 90.2 km/h |
| 65 mph (105 km/h) | 62.4 mph | 60.7 mph | 97.7 km/h |
| 70 mph (113 km/h) | 67.2 mph | 65.4 mph | 105.2 km/h |
| 75 mph (121 km/h) | 72.0 mph | 70.1 mph | 112.8 km/h |
What is Indicated Speed Calculator?
An Indicated Speed Calculator is a tool that predicts the speedometer reading at a known true road speed after a tire change. The tool answers the practical question: what number on the dash equals the speed limit.
The Indicated Speed Calculator is the inverse of the Actual Speed Calculator. One divides by the correction factor, and the other multiplies by it.
Drivers with taller tires hold an indicated speed below the posted limit. Drivers with shorter tires hold an indicated speed above the posted limit.
What are the uses of Indicated Speed Calculator?
There are 4 main uses of the Indicated Speed Calculator, including cruise control targets and dash stickers.
Cruise control targets
Set cruise control to the indicated value for each common limit.
Dash reference cards
Print a small card of indicated speeds for 30, 50 and 70 mph limits.
School zone compliance
Hold true speed at low limits where a few mph matter.
Driver training
Show new drivers of modified vehicles the speed gap.
Related Speedometer & Distance Tools
View All 134 Subtools →Estimate speed changes after tire resizing.
Estimate road speed from indicated speed.
Calculate a tire-size correction multiplier.
Tabulate indicated and estimated actual speeds.
Estimate distance recording differences.
Adjust indicated distance for tire changes.