Tire Size Calculator – Speedometer Error in 3 Easy Steps
Tire Size Calculator
Use this tire size calculator to compare tire dimensions and calculate exact speedometer error for wheel upgrades.
Speedometer Calibration
Larger tires rotate fewer times per mile, making you travel faster than the gauge reads.
Use this free tire size calculator to compare your original tires with a new size before you buy. Enter both sizes in the usual width/ratio R wheel format, and the calculator shows diameter, width, sidewall height and circumference side by side, the percentage change, and how fast you are really going when the speedometer shows a set speed.
Quick answer
Tire diameter = wheel size + 2 × (width × aspect ratio ÷ 100 ÷ 25.4) inches. A 225/45R17 tire is 24.97" tall, and a 245/40R18 is 25.72", a +2.98% change. At an indicated 60 mph, the car with the larger tire is really doing about 61.8 mph. Most tire shops recommend keeping the overall diameter within ±3% of the original.
Table of Contents
Bigger wheels and wider tires can change how your car looks and handles, but they also change the overall diameter. That affects your speedometer, odometer, gearing and sometimes ABS and traction control.
This tire size calculator shows every difference in one table and now flags any upgrade that goes beyond the common ±3% guideline.
How to Read a Tire Size
- 225 – tread width in millimetres.
- 45 – aspect ratio: the sidewall height is 45% of the width.
- R – radial construction.
- 17 – wheel (rim) diameter in inches.
Tire Size Formulas
- Sidewall (in) = width × ratio ÷ 100 ÷ 25.4
- Diameter (in) = wheel + 2 × sidewall
- Circumference (in) = diameter × π
- Actual speed = speedometer reading × new diameter ÷ original diameter
- Diameter change (%) = (new ÷ original − 1) × 100
Common Tire Size Comparisons
| Original | New | Original diameter | New diameter | Change | Real speed at 60 mph |
|---|---|---|---|---|---|
| 225/45R17 | 245/40R18 | 24.97" | 25.72" | +2.98% | 61.79 mph |
| 225/45R17 | 225/40R18 | 24.97" | 25.09" | +0.46% | 60.27 mph |
| 205/55R16 | 225/45R17 | 24.88" | 24.97" | +0.38% | 60.23 mph |
| 225/45R17 | 245/45R18 | 24.97" | 26.68" | +6.84% | 64.11 mph |
How to Use the Tire Size Calculator
Step 1: Enter the Original and New Tire Sizes
Fill in Original Tire (OEM) with the size printed on your current tire or the door-jamb sticker, for example 225 / 45 R 17. Then enter the size you are considering under New Tire (Upgrade), such as 245 / 40 R 18.
Step 2: Compare the Dimensions
The table compares both tires. The new tire is 0.74" taller (+2.98%), 20 mm wider, has a 0.13" shorter sidewall and a circumference 2.34" larger. Green values are increases and red values are decreases.
Step 3: Check the Speedometer and the ±3% Guideline
Enter a speed in Dashboard Reads to see your true speed. Try 245/45R18 instead: diameter rises by 6.84%, the real speed at an indicated 60 mph is 64.11 mph, and a red warning shows the size is outside the ±3% guideline.
Why the ±3% Rule Matters
- Speedometer: a taller tire makes the gauge read low, which can lead to speeding tickets.
- Odometer: fewer recorded miles than you actually drive.
- Safety systems: ABS, stability control and some driver aids compare wheel speeds.
- Clearance: larger tires may rub on suspension or wheel arches at full lock.
- Gearing: taller tires slightly reduce acceleration and change shift points.
Plus Sizing Explained
"Plus one" means going up one inch in wheel size while lowering the aspect ratio so the overall diameter stays nearly the same. For example, 205/55R16 to 225/45R17 is a plus-one change of only +0.38%. Always check the load index and speed rating of the new tire as well.
More automotive and unit tools: the Fuel Consumption Converter, the Tyre Pressure Bar to PSI Converter and the Length Converter.
Frequently Asked Questions
How do I calculate tire diameter?
Multiply width by aspect ratio, divide by 100 and by 25.4 to get sidewall height in inches, double it and add the wheel size. 225/45R17 = 24.97 inches.
How much can tire size change safely?
Most experts recommend staying within ±3% of the original overall diameter.
Do bigger tires make my speedometer read slower?
Yes. A taller tire covers more distance per turn, so you travel faster than the gauge shows.
Is 245/40R18 the same size as 225/45R17?
They are close: 245/40R18 is 2.98% taller, just inside the ±3% guideline.
Does tire width affect the speedometer?
Only through the sidewall height. With the same ratio, a wider tire has a taller sidewall and a larger diameter.
Is my data stored?
No. The tire size calculator runs in your browser and nothing is saved.