FORScan Tire Size Calculator
Calculate speedometer corrections when changing tire sizes for Ford, Lincoln, and Mercury vehicles using FORScan.
Enter the stock tire size from your vehicle’s door jamb sticker.
Enter the new, larger or smaller tire size you have installed.
Select your preferred unit for diameter and sidewall display. Circumference for FORScan is always in millimeters.
What is a FORScan Tire Size Calculator?
A forscan tire size calculator is a specialized tool designed for owners of Ford, Lincoln, and Mercury vehicles who use the FORScan software diagnostic tool. When you change the factory-installed tires to a different size (larger or smaller), your vehicle’s speedometer, odometer, and even transmission shift points become inaccurate. This is because the vehicle’s computer, specifically the Body Control Module (BCM), is calibrated for the original tire’s circumference. This calculator computes the precise circumference of your new tires in millimeters—the value required by FORScan—to recalibrate the BCM. By doing this, you ensure your speedometer reads accurately, your odometer logs correct mileage, and your automatic transmission shifts at the optimal points for performance and fuel efficiency.
The FORScan Tire Size Formula and Explanation
The calculations are based on the standard P-metric or Euro-metric tire code (e.g., 275/65R18). The formula involves converting these numbers into physical dimensions.
- Sidewall Height: This is calculated by multiplying the tire’s section width by its aspect ratio. `Sidewall Height = Width * (Aspect Ratio / 100)`
- Overall Diameter: This is the total height of the tire. It’s found by adding the sidewall height (twice) to the rim diameter. A crucial unit conversion is needed here since rim diameter is in inches. `Overall Diameter = (Sidewall Height * 2) + (Rim Diameter * 25.4)`
- Circumference: This is the most critical value for FORScan. It’s the total distance the tire travels in one full rotation. `Circumference = Overall Diameter * π (3.14159)`
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Section Width | The nominal width of the tire from sidewall to sidewall. | Millimeters (mm) | 185 – 355 |
| Aspect Ratio | The ratio of the sidewall’s height to the tire’s width. | Percentage (%) | 30 – 85 |
| Rim Diameter | The diameter of the wheel the tire is mounted on. | Inches (in) | 15 – 24 |
| Circumference | The rolling distance of one full tire rotation. This is the key value for FORScan. | Millimeters (mm) | 1800 – 3000 |
Practical Examples
Example 1: Upgrading a Ford F-150
- Inputs: Original Tire:
275/65R18, New Tire:295/70R18. - Units: Imperial for display.
- Results: The new tire has a circumference of approximately 2619 mm. The speedometer will be off by about +5.3%. When the speedometer shows 60 MPH, the actual speed will be ~63.2 MPH. This highlights the need for recalibration, a task for which our {primary_keyword} is perfect.
Example 2: Downsizing for Winter Tires on a Ford Explorer
- Inputs: Original Tire:
255/55R20, New Tire:245/65R18. - Units: Imperial for display.
- Results: The new, smaller winter tire has a circumference of approximately 2417 mm. The speedometer will read faster than the actual speed by about -1.1%. While a smaller error, using a tool like our {related_keywords} calculator ensures perfect accuracy.
How to Use This FORScan Tire Size Calculator
- Enter Original Tire Size: Input the tire size listed on your vehicle’s door jamb sticker into the “Original Tire Size” field.
- Enter New Tire Size: Input the size of your newly installed tires.
- Select Units: Choose between Imperial or Metric for display of diameter and sidewall height. Note: The final FORScan value will always be in millimeters.
- Calculate: Click the “Calculate” button.
- Interpret Results: The primary result is the “New Tire Circumference (for FORScan BCM)”. This is the number you need to enter into the BCM address 726-12-01 (or similar, check your vehicle’s specific address). The calculator also provides the hex equivalent, speedometer error, and a detailed comparison table.
Key Factors That Affect Tire Size Calculations
- Tire Wear: A worn tire has a smaller diameter and circumference than a new one. The calculations assume new tires.
- Inflation Pressure (PSI): Under-inflated tires will have a slightly smaller “rolling radius” than properly inflated ones. Always ensure tires are at the recommended pressure.
- Manufacturing Tolerances: Not all tires of the same size are identical. There can be slight variations between brands.
- Vehicle Load: The weight of the vehicle compresses the tires, slightly reducing the rolling radius. The calculated value is a very close approximation.
- GPS Verification: After using the calculator and programming FORScan, it’s best practice to verify your speedometer’s accuracy using a reliable GPS device.
- Correct BCM Address: Ensure you are using the correct BCM address line for your vehicle’s model and year. Using the wrong one can cause issues.
Frequently Asked Questions (FAQ)
- What value do I enter into FORScan?
- You need to enter the calculated tire circumference in millimeters into the appropriate BCM address. This calculator provides that exact value.
- What is the BCM address for tire size?
- For many Ford models (like the F-150 from 2015-2020), the common address is 726-12-01. However, this can vary by model and year. Always consult a reliable FORScan guide for your specific vehicle.
- Do I need to convert the circumference to hex?
- Some older FORScan interfaces or specific modules may require a hexadecimal value. This calculator provides both the decimal (mm) and the hex value for convenience. Most modern versions of FORScan allow direct entry of the millimeter value.
- Will this fix my transmission shift points?
- Yes. The transmission control module (TCM) uses vehicle speed to determine when to shift gears. Correcting the speedometer with this {primary_keyword} also provides the TCM with the accurate speed, leading to smoother and more efficient shifting.
- How accurate is this calculator?
- It is highly accurate for theoretical calculations based on tire size numbers. However, real-world factors like tire pressure and manufacturing differences can cause minor deviations. Using a {related_keywords} tool after calibration is recommended.
- What happens if I don’t recalibrate my speedometer?
- Your speedometer and odometer will be inaccurate, you may experience incorrect transmission shifting, and your vehicle’s electronic stability control (ESC) system may not function as designed.
- Can I use this for any vehicle?
- This calculator is specifically designed for vehicles that can be programmed with FORScan, primarily Ford, Lincoln, and Mercury models. For other vehicle tools, see our section on related resources like our {related_keywords} page.
- Is a PCM/TCM relearn necessary?
- Yes, after writing the new tire size to the BCM, it is highly recommended to perform a “Module initialization/Relearn vehicle data” procedure for the PCM and TCM to ensure all modules are using the new data and to clear any potential diagnostic trouble codes (DTCs).
Related Tools and Internal Resources
For more advanced automotive calculations, check out these other resources:
- Gear Ratio Calculator: Determine your effective gear ratio after changing tire sizes.
- Speedometer Error Calculator: A standalone tool to quickly check your speedometer error based on two tire sizes.
- 4 Link Calculator: For advanced suspension setup and analysis. Check our guide on {related_keywords}.
- RPM Calculator: Calculate engine RPM at any speed based on tire size and gear ratios.
- Wheel Offset Calculator: Understand how new wheels will fit on your vehicle.
- MPG Calculator: An important tool for any vehicle owner, discover our {related_keywords} for fuel efficiency.