Actual Speed Calculator
Tire Size Speed Calculator
The “Actual Speed Calculator” that allows you to input the new tire diameter, OEM (original equipment manufacturer) tire diameter, and speedometer reading in order to calculate your actual speed based on tire size changes.
Measure Your New Tire Diameter
To begin, you’ll need to accurately measure the diameter of your new tires in inches. This can be done by using a tape measure to measure from the ground to the top of the tire, going through the center point.
Be sure to measure in a straight line and not along the curve of the tire. Record this measurement, as it will be needed to input into the calculator.
Determine Your OEM Tire Diameter
Next, you need to know the diameter of the original tires that came with your vehicle from the manufacturer (OEM tire diameter).
This information can usually be found in your vehicle’s owner’s manual or on the driver’s side door jamb placard. The tire size will be in a format such as P215/60R16, where the last number (16 in this example) represents the wheel diameter in inches. Record the OEM tire diameter.
Take Note of Your Speedometer Reading
When you’re ready to use the calculator, take your vehicle for a test drive on a flat, straight road. Once at a steady speed, take note of the speedometer reading in miles per hour (mph).
It’s important that this reading is taken while driving with your new tires installed. Record the speedometer reading.
Input Values Into the Actual Speed Calculator
With your three measurements recorded, you’re ready to use the online calculator. In the “New Tire Diameter (inches)” field, input the measurement you took in Step 1.
Then, input the OEM tire diameter determined in Step 2 into the “OEM Tire Diameter (inches)” field. Finally, enter your noted speedometer reading from Step 3 into the “Speedometer Reading (mph)” field.
Click “Calculate” To Determine Your Actual Speed
After inputting all three values, click the blue “Calculate” button. The calculator will then determine your actual driving speed based on the tire size change.
This actual speed takes into account the effect of the larger or smaller tire diameter on your speedometer reading.
Interpret the results
The calculator’s result will display your actual speed in comparison to what your speedometer is reading. For example, if your new tires are larger than the OEM tires, your actual speed will be lower than what your speedometer shows.
This means you are driving slower than your speedometer indicates. Conversely, if your new tires are smaller than the OEM, your actual speed will be higher than your speedometer reading. Keep this difference in mind when driving, as it may affect your adherence to posted speed limits.
By using the Actual Speed Calculator and following these steps, you can easily determine how tire size changes impact your vehicle’s speedometer accuracy and actual driving speed. This information can help you make informed decisions about tire size and maintain safe driving habits.