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Tire Size Impact Calculator

This calculator determines how an aftermarket tire’s width, aspect ratio and rim size alter its overall circumference. Using the standard formula for tread diameter—rim diameter plus twice the sidewall height—it computes the new overall diameter in millimeters and inches. The speedometer error is derived from the ratio of new to old diameters; a larger wheel causes the odometer to under‑read speed by that percentage, while a smaller wheel over‑reads it. A rough fuel‑economy estimate assumes MPG scales inversely with overall circumference: a 1 % increase in diameter predicts roughly a 1 % drop in MPG.

Enter the width of your current tire in millimeters.
The sidewall height as a percentage of width for your current tire.
Diameter of the rim on which you currently mount the tire, in inches.
Enter the width of the aftermarket tire you are considering, in millimeters.
Sidewall height as a percentage of width for the new tire.
Diameter of the rim on which the new tire will mount, in inches.
Vehicle’s miles‑per‑gallon rating with its stock tires.

What it is

The Tire Size Impact Calculator is a precision tool that lets you evaluate how changing the width, aspect ratio or rim size of an aftermarket tire will alter its overall circumference and, consequently, your vehicle’s speedometer accuracy, estimated fuel economy and handling characteristics. By computing the new overall diameter from both mm and inches and comparing it to the stock diameter, the calculator derives a speed‑error percentage that tells you how much your odometer will over‑ or under‑report real speed. It also provides a best‑effort MPG estimate assuming mileage inversely tracks tire circumference, giving buyers a clearer sense of how wheel upgrades translate into fuel cost and driving dynamics.

How to use it

Enter each existing tire specification—width (mm), aspect ratio (%), and rim diameter (in)—as well as the corresponding measurements for your aftermarket option. Input your vehicle’s baseline MPG to obtain an estimated new mileage figure. The results will display: the new overall diameter in both mm and inches, the speedometer‑error percentage, the revised MPG estimate, and the relative fuel‑economy change. Use the speed error to gauge how much your odometer reading will shift with the new wheels, and use the fuel change to predict additional fuel cost or saving.

Worked example

1. Compute the old diameter: rim 16 in ×25.4 mm/in = 406.4 mm; sidewall height = width 205 mm ×55 % ÷100 =112.75 mm, double =225.5 mm. Sum = 631.9 mm. 2. Compute the new diameter: rim 17 in ×25.4=431.8 mm; sidewall height =225×40 %=90 mm, double =180 mm. Total = 611.8 mm. 3. Speed‑error percentage: (611.8 ÷631.9 –1)×100 = –3.181 % → the speedometer will under‑read by about 3.2 %. 4. Estimated MPG: baseline 28 ×(631.9 ÷611.8) = 28.92 mpg (≈3 % higher mileage). 5. Fuel economy change: (28.92–28)/28×100 ≈ +3.285 % → roughly a 3.3 % improvement in fuel efficiency.

Inputs

  • Old Tire Width (mm): 205
  • Old Aspect Ratio (%): 55
  • Old Rim Diameter (in): 16
  • New Tire Width (mm): 225
  • New Aspect Ratio (%): 40
  • New Rim Diameter (in): 17
  • Baseline MPG: 28

Result

  • New Overall Diameter (mm): 611.8
  • New Overall Diameter (in): 24.087
  • Speedometer Error (%): -3.181
  • Estimated MPG: 28.92
  • Fuel Economy Change (%): 3.285

Frequently asked questions

What happens if I enter the same tire sizes for old and new wheels?

All impact metrics will report zero, indicating no change in speedometer accuracy or fuel economy.

How reliable are the fuel‑economy estimates?

They are based on a simple inverse relationship between circumference and MPG. Real world results may vary due to rolling resistance, vehicle load, tire pressure and driving style.

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