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Ubify Financial Advisory Board
FINANCIAL ANALYSTS & CPA REVIEWERS
About This Tool
Upgrading to aftermarket wheels, plus-sizing alloy rims, or fitting larger all-terrain tires alters vehicle rolling geometry, ride height, suspension clearance, and speedometer calibration. This comparison studio calculates dimensional variances between OEM stock wheels and target aftermarket fitments, measuring diameter shifts, sidewall changes, and true vehicle speed.
### Tire Sizing Mathematics Tire dimensions follow standardized metric tire code notation (e.g., `225/45R17`): - **Sidewall Height (Inches):** Calculated from section width and aspect ratio: `sidewallInches = (widthMm × (aspectRatio / 100)) / 25.4`. - **Overall Wheel Diameter:** Combines wheel rim diameter with two opposing sidewalls: `overallDiameter = rimDiameter + (2 × sidewallInches)`. - **Tire Circumference:** `circumference = overallDiameter × \pi`. - **Revolutions Per Mile:** `revsPerMile = 63,360 / circumference`. - **Speedometer Calibration Error:** Because vehicular transmission sensors measure axle rotations rather than ground speed, altering rolling circumference produces proportional speedometer deviation: `actualSpeed = indicatedSpeed × (newDiameter / stockDiameter)`.
### Worked Comparison Scenario Comparing factory stock `225/45R17` tires against plus-sized `245/40R18` performance tires at a reference 65 MPH speed: 1. **Stock 225/45R17:** Sidewall = 3.99 inches; Overall Diameter = `17 + (2 × 3.99) = 24.97 inches`; Circumference = 78.45 inches; Revs/mile = 808. 2. **New 245/40R18:** Sidewall = 3.86 inches; Overall Diameter = `18 + (2 × 3.86) = 25.72 inches`; Circumference = 80.79 inches; Revs/mile = 784. 3. **Dimensional Variance:** Diameter increases by `+0.75 inches (+3.0%)`. 4. **Speedometer Impact:** When your vehicle dashboard reads 65.0 MPH, actual road speed is `66.9 MPH` (+1.9 MPH faster than indicated).
### Vehicle Fitment Privacy All automotive geometry calculations, sidewall transformations, and speedometer offset equations execute 100% locally in your web browser. No vehicle registration details, tire sizes, or speed telemetry are uploaded to external servers.
How to Use
- 1
Enter your vehicle’s factory tire specs: Stock Width (mm), Aspect Ratio (%), and Rim Diameter (in).
- 2
Enter your proposed new tire specs: New Width (mm), Aspect Ratio (%), and Rim Diameter (in).
- 3
Set your reference highway cruising speed in Reference Speedometer (MPH).
- 4
Inspect the side-by-side dimension comparison table: Sidewall Height, Overall Diameter, Circumference, and Revs per Mile.
- 5
Check the true ground speed at your reference speed to see speedometer calibration variance.
Frequently Asked Questions
What is the acceptable diameter difference when upsizing tires?
Most automotive manufacturers and tire technicians recommend keeping overall tire diameter changes within ±3% of OEM factory specification to prevent suspension rubbing and transmission sensor errors.
Will larger tires make my speedometer read faster or slower than actual speed?
Larger diameter tires cover more distance per wheel rotation. As a result, your speedometer will read slower than your actual ground speed (e.g., reading 65 MPH when traveling 67 MPH).
Are my vehicle specs transmitted across the web?
No. All tire sizing and speedometer math runs locally in your client web browser with zero server data storage.