FORScan Tire Size Calculator
Correct your Ford vehicle’s speedometer and transmission settings after changing tire sizes. This forscan tire size calculator provides the exact values needed for the Body Control Module (BCM) in FORScan. Get accurate decimal and hexadecimal values to ensure your truck or car runs perfectly with its new wheels.
Original Tire Size
New Tire Size
Calculator Results
Visual Comparison
| Metric | Original Tire | New Tire | Difference |
|---|---|---|---|
| Diameter (in) | … | … | … |
| Width (in) | … | … | … |
| Sidewall (in) | … | … | … |
| Circumference (in) | … | … | … |
| Revs 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 diagnostic tool. When you install tires that are a different size from the factory originals, your vehicle’s onboard computers—specifically the Body Control Module (BCM) and Powertrain Control Module (PCM)—no longer have the correct data for calculations. This leads to an inaccurate speedometer, incorrect odometer readings, and can even affect transmission shift points and ABS braking performance. This calculator computes the exact tire circumference value that needs to be programmed into the BCM to recalibrate these systems for your new tire size.
This tool is essential for anyone modifying their truck or car with larger off-road tires, different wheel sizes, or simply non-stock tire dimensions. Unlike a generic tire size calculator, a forscan tire size calculator provides the specific output (both decimal and hexadecimal) required by the FORScan software for address 726-12-01, making it a critical step in the vehicle modification process.
FORScan Tire Size Formula and Mathematical Explanation
The core calculation revolves around finding the new tire’s circumference in millimeters. Vehicle systems rely on knowing the precise distance the tire travels in one full revolution to accurately calculate speed and distance. Here is the step-by-step mathematical breakdown used by our forscan tire size calculator.
- Calculate Sidewall Height: The tire’s sidewall height is a percentage of its width. The formula is: `Sidewall Height (mm) = Tire Width (mm) * (Aspect Ratio / 100)`.
- Calculate Overall Tire Diameter: The total diameter is the wheel diameter plus two sidewall heights (one above the wheel, one below). Since rim diameter is in inches, it must be converted to millimeters. The formula is: `Diameter (mm) = (Rim Diameter (in) * 25.4) + (2 * Sidewall Height (mm))`.
- Calculate Tire Circumference: This is the final value needed for FORScan. It’s the diameter multiplied by Pi (π). `Circumference (mm) = Diameter (mm) * 3.14159`.
- Convert to Hexadecimal (Optional): For some “As-Built” programming methods in FORScan, this decimal value must be converted to a 4-digit hexadecimal number. Our calculator does this for you automatically.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Tire Width | The nominal width of the tire’s cross-section. | mm | 215 – 315 |
| Aspect Ratio | The sidewall height as a percentage of the width. | % | 35 – 75 |
| Rim Diameter | The diameter of the wheel it’s mounted on. | inches | 16 – 22 |
| Circumference | The target value for FORScan BCM programming. | mm | 2000 – 3000 |
Practical Examples (Real-World Use Cases)
Example 1: Upgrading a Ford F-150 to Larger Off-Road Tires
- Stock Tires: 275/65R18
- New Tires: 35×12.50R18 (equivalent to approx. 318/65R18)
- Input into Calculator: New Width=318, Aspect Ratio=65, Rim=18.
- Calculator Output: The forscan tire size calculator would provide a new circumference value of approximately 2786 mm (or 0AEA in hex).
- Interpretation: The user would connect to their F-150 with FORScan, navigate to the BCM configuration, and enter `2786` into the tire circumference field. This would correct the speedometer, which would otherwise read about 10% too slow.
Example 2: Changing to Low-Profile Tires on a Ford Mustang
- Stock Tires: 255/40R19
- New Tires: 275/35R19
- Input into Calculator: New Width=275, Aspect Ratio=35, Rim=19.
- Calculator Output: The calculator would determine the new circumference is slightly smaller, around 2105 mm (0839 hex).
- Interpretation: Even a small change requires correction. The user programs this value to prevent the speedometer from reading artificially high and to keep the odometer accrual accurate. Using a FORScan programming guide is recommended for the first time.
How to Use This FORScan Tire Size Calculator
- Enter Original Tire Size: In the “Original Tire Size” section, input the width, aspect ratio, and rim diameter from your vehicle’s factory tires. This establishes a baseline.
- Enter New Tire Size: In the “New Tire Size” section, input the dimensions of your newly installed tires.
- Review the Results: The calculator instantly updates. The most important output is the “New FORScan BCM Value” displayed in the green box. Both decimal and hexadecimal values are provided.
- Program with FORScan: Connect the FORScan software to your vehicle with a compatible OBD2 adapter. Navigate to the BCM (Body Control Module) configuration page. Find the entry for “Tire Circumference” (often in plain English, not As-Built) and type in the decimal value from the calculator. Write the change to the module.
- Verify: Use a GPS-based speedometer app on your phone to verify that your vehicle’s speedometer now matches the GPS speed. Minor adjustments of a few millimeters up or down in FORScan may be needed to get it perfect, as explained in our guide to understanding tire specs.
Using a reliable forscan tire size calculator like this one removes all guesswork from the process.
Key Factors That Affect FORScan Tire Size Results
- Actual vs. Nominal Size: The numbers on a tire’s sidewall are nominal. The true, measured diameter can vary slightly between manufacturers. For ultimate precision, it is best to measure the tire’s rolling circumference yourself, but this calculator gets you extremely close.
- Tire Pressure: Higher or lower tire pressure slightly changes the rolling diameter of the tire, which in turn affects the circumference. Calibrate your speedometer at your normal operating tire pressure.
- Vehicle Load: The weight of the vehicle compresses the tire, reducing the rolling radius. The formula used here is a standardized calculation, but heavy loads can cause minor discrepancies.
- Tire Wear: As your tires wear down, their diameter decreases. A brand-new tire will have a larger circumference than one at the end of its life. The impact is usually small but can account for a 1-2% difference over time.
- BCM vs. PCM: The proper way to change tire size is in the BCM. The BCM then communicates this to the PCM and other modules. Directly changing values in the PCM can cause conflicts.
- Correct FORScan Address: You must ensure you are writing to the correct BCM address. For most modern Fords, there’s a plain-text configuration option for “Tire Circumference,” which is safer than editing the “As-Built” hexadecimal data directly (e.g., at 726-12-01). Our tool, a dedicated forscan tire size calculator, gives you the value for that plain-text field.
Frequently Asked Questions (FAQ)
What is FORScan?
FORScan is a powerful software application that allows users to perform diagnostics and configure modules on Ford, Mazda, Lincoln, and Mercury vehicles. It requires a compatible OBD2 adapter to connect a computer to the vehicle.
Why do I need to recalibrate my speedometer?
Your vehicle calculates speed based on how fast the wheels are turning multiplied by the factory tire circumference. If you install larger tires, the wheels turn slower for a given speed, causing your speedometer to read lower than your actual speed. The opposite is true for smaller tires.
Is using a forscan tire size calculator dangerous?
The calculator itself is perfectly safe. The risk comes from improperly using the FORScan software. Always back up your vehicle’s original module data before making any changes. Typing the wrong value can cause issues, so double-check your numbers. Using the plain-text configuration is much safer than editing “As-Built” data.
What happens if I don’t correct the tire size?
Besides an incorrect speedometer, your odometer will record mileage inaccurately, affecting resale value. More importantly, automatic transmission shift points can become erratic, and the Anti-lock Braking System (ABS) and stability control may not function as designed in emergencies.
What is the difference between decimal and hexadecimal values?
Decimal is the base-10 number system we use daily. Hexadecimal is a base-16 system used in computing. Older “As-Built” programming methods in FORScan required the hexadecimal value. Modern versions often have a plain-text field where you can just enter the decimal circumference in millimeters, which this forscan tire size calculator provides.
Can the dealer do this for me?
Some dealers will, but many are reluctant to perform modifications outside of factory specifications. They may also charge a significant labor fee for what is a five-minute job with FORScan. Check out our gear ratio calculator for other driveline adjustments.
Does this work for all Ford models?
This method works for a wide range of modern Ford vehicles, especially trucks like the F-150, Ranger, and Super Duty, as well as cars like the Mustang and Explorer. Always confirm compatibility for your specific vehicle year and model in FORScan forums.
What OBD2 adapter should I use?
It is critical to use a high-quality adapter recommended by the FORScan team, such as the OBDLink EX or OBDLink MX+. Cheap, generic adapters can be unreliable and may fail during a module write, potentially damaging the module.
Related Tools and Internal Resources
For more in-depth vehicle modifications and calculations, explore our other tools and guides. Proper setup ensures your vehicle runs safely and efficiently.
- Wheel Offset Calculator – Understand how wheel offset and backspacing affect your vehicle’s stance and suspension clearance.
- Beginner’s Guide to Using FORScan – A step-by-step tutorial on getting started with the FORScan software, from installation to making your first changes. This is a great resource if you’re new to the platform.
- Ford F-150 VIN Decoder – Decode your vehicle’s VIN to get detailed information about its factory specifications, engine, and trim level.
- Best Performance Mods for the Ford Raptor – Discover the most effective upgrades for your Ford Raptor, from suspension and exhaust to tuning and tire choices.