> ## Documentation Index
> Fetch the complete documentation index at: https://help.tiretutor.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# How to Use the Custom Fitment Tool

> Use this when a customer wants to change their rim size (for example, upgrading to larger rims, adding chunkier off-road tires, or fitting skinnier winter tires) and you need to confirm the new wheel-and-tire combination will fit their vehicle safely.

## A plus-size calculator, right inside the Shop Management System

<Frame>
  <img src="https://mintcdn.com/tiretutor/KKLx568cSj-lc6K5/images/resources/how-to/how-to-use-the-custom-fitment-tool-1-desktop.jpg?fit=max&auto=format&n=KKLx568cSj-lc6K5&q=85&s=78c82654ec1b9c3b4e46318fb7a90dd2" alt="The Plus Size Calculator screen, reached from Fitment in the left-hand menu. Enter Tire Size holds 2256517, Select Tolerance OD is 3% and Select Tolerance Section Width is 20mm, beside a Calculate button. The results table groups matches by rim size, with columns Tire Size, Measurement, Error Percent, OD, Indicated mph and Mph Error — under Rim Size 17.5 inch, offset plus 0.5, the row 205/65-17.5 reads -1.9%, OD 28, 58.9 mph and -1.1 mph." width="1330" height="845" data-path="images/resources/how-to/how-to-use-the-custom-fitment-tool-1-desktop.jpg" />
</Frame>

Use this when a customer wants to change their rim size (for example, upgrading to larger rims, adding chunkier off-road tires, or fitting skinnier winter tires) and you need to confirm the new wheel-and-tire combination will fit their vehicle safely.

**1. Open the Fitment tool.** From the left-hand menu, go to Shop Settings, then select Fitment. (You can also access Shop Settings from the icon at the top of the screen.)

**2. Enter the customer's current tire size.** This is your starting point — the tool uses it to calculate compatible alternatives.

**3. Set your tolerance percentage.** This controls how much variation in overall diameter you'll allow, based on the space available in the vehicle's wheel well.

**4. Set the width allowance.** Specify how much wider than the original tire you're willing to go.

**5. Click Calculate.** The tool will return a list of compatible rim-and-tire combinations, including speed ratings for each option.

**6. Select the combination the customer wants.** The selected option will carry over into the catalog so you can add it directly to a work order or purchase.

**Why this matters:** If you skip this step and fit a tire whose overall diameter is off-spec, you can throw off the vehicle's speedometer, odometer, ABS, and traction control. A tire that scrapes the fender well will also cause premature wear. In some states, a significantly off-spec odometer reading is a legal issue at resale.