Tire Size Comparison Calculator: Compare Two Sizes Side by Side
Compare two tire sizes such as 225/45R17 and 245/40R18: overall diameter, sidewall height and how far the speedometer reading would be off. Reference only.
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Nothing is sent until you press Send. Your measurements are included only if you tick the box, and you can see the full report below first.
When to use this
- You are thinking about a different tire size or a plus-one wheel upgrade and want to see how the overall diameter changes.
- You want to know how far off the speedometer would read with the new size.
- You want a quick side-by-side picture of the width, sidewall and overall diameter of two sizes.
How to use
- Read the size printed on your tire sidewall, for example 225/45R17, and type it as the current size.
- Type the size you are considering as the replacement size.
- Choose mph or km/h and press Compare tire sizes.
- Read the diameter change, the speedometer table and the drawing. Copy the result if you want to share it.
Examples
- 225/45R17 to 245/40R18: The first tire has a sidewall of 101.25 mm and an overall diameter of 634.3 mm. The second has 98 mm and 653.2 mm, 2.98 percent larger. With the speedometer showing 60 mph, the true speed is about 61.8 mph.
Understanding the result
The sidewall height is the width times the aspect ratio divided by 100. The overall diameter is the rim diameter in millimetres (inches x 25.4) plus two sidewalls. A larger diameter means the tire covers more road per turn, so the true speed is the shown speed multiplied by the new diameter divided by the old one, assuming the speedometer read correctly before.
Limitations
- Reference calculation only. Not safety or legal advice.
- The tool does not check load rating, speed rating, wheel width, offset, clearance or legal requirements. Ask a qualified tire professional and check your vehicle's placard.
- Only metric sizes such as 225/45R17 are read. Flotation sizes like 31x10.5R15 are not.
- A different overall diameter also affects gearing, braking and stability control systems.
FAQ
How do I compare two tire sizes?
Work out the overall diameter of each: rim size in millimetres plus twice the sidewall height, where the sidewall is width times the aspect ratio divided by 100. This tool does it and shows the difference.
How much will my speedometer be off with bigger tires?
It reads low by about the same percentage as the diameter increase. A 3 percent larger tire means that a shown 60 mph is really about 61.8 mph.
What do the numbers in 225/45R17 mean?
225 is the tread width in millimetres, 45 is the sidewall height as a percentage of the width, R means radial construction and 17 is the rim diameter in inches.
Is a 3 percent difference acceptable?
Many sources use about 3 percent as a guide, but this is not a rule for your vehicle. Check with a tire professional and your vehicle placard.
Does this tell me whether the tire will fit?
No. Fit depends on wheel width, offset, suspension and clearance, which this tool does not know. It is a reference calculation only.
Is my information saved anywhere?
No. Everything is calculated in your browser and nothing is sent to a server. The share link keeps your sizes after the # sign.
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