Forscan Tire Size Calculator

Recalibrate your Ford vehicle’s speedometer and Body Control Module (BCM) after changing tire sizes. This calculator generates the correct hexadecimal value needed for FORScan software.

Stock Tire Size

New Tire Size

Tire Size Comparison

Metric	Stock Tire	New Tire
Overall Diameter (in)	—	—
Sidewall Height (in)	—	—
Circumference (in)	—	—
Revolutions per Mile	—	—

What is a FORScan Tire Size Calculator?

A forscan tire size calculator is a specialized tool designed for owners of Ford, Lincoln, and Mazda vehicles who use the FORScan software. When you install larger or smaller tires on your truck or SUV, the vehicle’s computer (specifically the Body Control Module or BCM) continues to operate based on the original factory tire dimensions. This leads to inaccuracies in your speedometer reading, odometer, and potentially affects shift points and traction control systems. A forscan tire size calculator precisely computes the necessary hexadecimal value that must be programmed into the BCM to recalibrate it for the new tire size. This ensures your vehicle’s systems have the correct data to function accurately and safely.

This tool is essential for anyone who has modified their vehicle with aftermarket tires, such as off-road enthusiasts upgrading a Ford F-150. Without using a proper forscan tire size calculator, your speedometer could be off by a significant margin, leading to unintentional speeding or incorrect mileage tracking.

FORScan Tire Size Formula and Mathematical Explanation

The core of the forscan tire size calculator lies in a multi-step calculation that converts standard tire measurements into a hexadecimal value for the BCM. The process is as follows:

1. Calculate Sidewall Height: The height of the tire’s sidewall is determined by multiplying the tire’s width by its aspect ratio. Sidewall Height (mm) = Tire Width (mm) * (Aspect Ratio / 100)
2. Calculate Overall Diameter: The total diameter of the tire is the wheel diameter plus two times the sidewall height. Since wheel diameter is in inches, it must be converted to millimeters (1 inch = 25.4 mm). Overall Diameter (mm) = (Sidewall Height * 2) + (Wheel Diameter * 25.4)
3. Calculate Circumference: The circumference is the total distance the tire travels in one revolution. Circumference (mm) = Overall Diameter * π (Pi ≈ 3.14159)
4. Convert to Hexadecimal: This is the key step for FORScan. The calculated circumference in millimeters is rounded to the nearest whole number and then converted into a four-digit hexadecimal string. For example, a circumference of 2588 mm becomes `0A1C` in hex. This is the value our forscan tire size calculator provides.

Calculation Variables

Variable	Meaning	Unit	Typical Range
Tire Width	The width of the tire from sidewall to sidewall.	mm	245 – 315
Aspect Ratio	The ratio of the tire’s height to its width.	%	40 – 75
Wheel Diameter	The diameter of the rim the tire is mounted on.	inches	17 – 22
Circumference	The distance covered in one tire revolution.	mm	2200 – 2800

Practical Examples (Real-World Use Cases)

Example 1: Upgrading a Ford F-150 for Off-Roading

An owner of a Ford F-150 with stock 275/65R18 tires decides to install larger 295/70R18 all-terrain tires for better off-road performance. Using the forscan tire size calculator:

• Inputs: Stock (275/65R18), New (295/70R18)
• Outputs: The calculator determines the new tire circumference is approximately 2697 mm.
• FORScan Value: The hex value generated is `0A89`.
• Interpretation: The owner connects to their truck with FORScan, navigates to the BCM module (address 726-12-01), and writes the new `0A89` value. After a module reset, the speedometer now reads accurately, and the odometer logs the correct mileage. This is a perfect example of a how to change tire size with FORScan procedure.

Example 2: Correcting Speedometer After Buying a Used Truck

Someone buys a used Ford Ranger that already has oversized tires, and they notice via GPS that the speedometer is reading 5 MPH slow at highway speeds. They use the forscan tire size calculator to determine the correct value.

• Inputs: They find the original tire size on the door jamb sticker (e.g., 265/60R18) and enter the current oversized tire size (e.g., 285/65R18).
• Outputs: The calculator provides the hex value for the 285/65R18 tires.
• Interpretation: By programming this value, they perform a speedometer correction after new tires were installed by the previous owner, ensuring they are driving at the legal speed limit and that the truck’s systems function as intended.

How to Use This FORScan Tire Size Calculator

1. Enter Stock Tire Size: In the “Stock Tire Size” section, input the Width, Aspect Ratio, and Wheel Diameter of the tires that came from the factory. You can find this on the sticker inside your driver-side door jamb.
2. Enter New Tire Size: In the “New Tire Size” section, input the dimensions of your newly installed tires.
3. Get the FORScan Value: The calculator will instantly update and display the 4-digit hexadecimal “New FORScan BCM Value.” This is the primary result you need.
4. Review Key Metrics: The forscan tire size calculator also shows the new tire’s diameter, circumference, and the percentage of speedometer error before correction. A positive error means your speedometer reads slower than your actual speed.
5. Program with FORScan: Connect to your vehicle using an OBD2 adapter and the FORScan software. Navigate to the BCM (As-Built) configuration, find address `726-12-01`, and replace the second block of four characters with the value from the calculator. Write the changes and cycle the ignition. A BCM tire size calculator is the most reliable way to get this value.

Key Factors That Affect FORScan Tire Size Results

Several factors can influence the outcome when using a forscan tire size calculator. Accuracy is key for safety and performance.

• Tire Wear: A heavily worn tire has a smaller diameter than a new one of the same size. For maximum accuracy, it’s best to perform calculations with new tires.
• Tire Inflation (PSI): Tire pressure affects the “squash factor” or loaded radius of the tire. The calculations assume proper inflation. Under-inflated tires will have a slightly smaller effective circumference.
• Manufacturing Variances: Not all tires are created equal. A 285/70R18 from one brand may have a slightly different actual diameter than the same size from another brand. The forscan tire size calculator uses the nominal size, which is extremely close for most applications.
• Vehicle Load: The weight in your vehicle compresses the tires, slightly reducing the distance from the axle to the ground. This has a minor but measurable effect on revolutions per mile.
• Correct BCM Address: You must ensure you are writing the value to the correct address in FORScan (typically 726-12-01 for modern F-series trucks). Writing to the wrong address can cause vehicle issues.
• OBD2 Adapter Quality: A stable connection is critical when writing data to your vehicle’s modules. Using a high-quality, recommended OBD2 programming for tire size adapter is essential to prevent data corruption.

Frequently Asked Questions (FAQ)

1. What is FORScan and do I need it?

FORScan is a software application that allows you to access and modify the configuration of your Ford, Mazda, or Lincoln vehicle’s onboard computer modules. You need it, along with a compatible OBD2 adapter, to apply the hexadecimal value generated by this forscan tire size calculator.

2. Will using a forscan tire size calculator void my warranty?

Altering your vehicle’s module configuration can potentially void the warranty for related components if a failure is traced back to the changes you made. Many dealerships are accustomed to these changes for tire sizes, but it’s best to proceed with caution. You can always save your original BCM value and revert back if needed.

3. What happens if I enter the wrong value?

Entering an incorrect value will result in an inaccurate speedometer and odometer. As long as you only change the tire size value at the specified address, it is generally safe and reversible. Simply re-calculate and write the correct value.

4. Why is the FORScan value in hexadecimal?

Vehicle control modules use hexadecimal numbers for data storage as it’s a more compact and efficient way to represent binary data compared to decimal numbers. This forscan tire size calculator handles the conversion for you.

5. My speedometer is still off after using the calculator. Why?

First, double-check that you entered the correct tire sizes. Second, verify the value was written correctly in FORScan. If it’s still slightly off, it may be due to tire wear or brand-specific size variations. Some users fine-tune the value by comparing their speedometer to a GPS device and making small adjustments to the circumference value before converting to hex.

6. Does this calculator work for all Ford models?

This calculator is primarily designed for modern Ford F-Series trucks (F-150, F-250, etc.), Rangers, and Expeditions where the BCM stores the tire circumference in millimeters at address 726-12-01. It may work for other models, but you must verify the correct BCM address for your specific vehicle.

7. Can I use this for gearing changes?

No, this forscan tire size calculator is only for tire size changes. Recalibrating for differential gear changes requires modifying a different module and address within FORScan (often the PCM).

8. What if I have a lift kit?

A lift kit itself doesn’t require a BCM change. However, people install lift kits to fit larger tires. The recalibration is for the tires, not the lift. Be sure to check a Ford truck lift and tire guide to ensure your desired tire size fits with your lift.