Tire Size Calculator
Compare the overall diameter and speedometer difference between two tire sizes (width/aspect ratio/rim diameter).
- Tire 125.86 in
- Tire 226.33 in
Tire 2 stands 1.81% taller — that changes how far you actually travel per wheel rotation versus what your speedometer reads.
- Tire 1 Diameter25.86 in
- Tire 2 Diameter26.33 in
This difference is small enough to be within the typical acceptable range.
About the Tire Size Calculator
This calculator compares the overall diameter of two tire sizes given as width/aspect-ratio/rim-diameter — the three numbers printed on a tire's sidewall, like 225/50R17. It's built for the moment you're eyeing a size change and want to know how much it actually shifts the tire's height before you order a set.
It's used for plus-sizing (moving to a wider tire on a bigger wheel while trying to keep overall diameter close to stock), for checking whether a used set of tires from a different size will roughly fit, and for understanding how a size swap will throw off a speedometer or odometer reading.
Enter both sizes and it works out each tire's overall diameter along with the percentage difference between them — the number that actually matters for fitment and speedometer accuracy.
How it’s calculated
Overall diameter starts from the rim diameter (already in inches) and adds twice the sidewall height, since the sidewall appears above and below the rim. Sidewall height is the tire's width in millimeters times its aspect ratio (as a percentage), converted from millimeters to inches by dividing by 25.4.
A tire's speedometer reading assumes the diameter (and therefore the distance covered per revolution) the vehicle was calibrated for. Swap in a tire with a meaningfully different overall diameter and every wheel rotation covers a different distance than the speedometer expects, throwing off both the speed reading and the odometer's mileage count by roughly the same percentage as the diameter difference.
Frequently asked questions
How much tire size difference is considered safe?
A commonly cited rule of thumb is to keep overall diameter within about 3% of the stock size — beyond that, speedometer error, clearance issues, and handling changes become more noticeable. Some vehicles tolerate more, some less, so check clearance physically when in doubt.
What do the three numbers on a tire (like 225/50R17) mean?
225 is the tire's width in millimeters, 50 is the aspect ratio — the sidewall height as a percentage of that width — and 17 is the wheel rim diameter in inches that the tire is built to fit.
Why would a larger tire make my speedometer read slower than my actual speed?
A taller tire covers more ground per revolution than the vehicle's speed sensor was calibrated to expect, so the speedometer under-reads your actual road speed — you're going faster than the dash shows.
Does plus-sizing (bigger wheel, lower-profile tire) change the overall diameter?
Not necessarily — the goal of plus-sizing is usually to pick a lower aspect ratio tire on a larger wheel so the overall diameter stays close to the original, which is exactly the comparison this calculator is built to check.
Related calculators
Powered by GetCalculator.online