The calculation, without hidden assumptions
Changing tire size changes more than width. The sidewall ratio changes overall diameter, which changes distance traveled per wheel revolution. This calculator compares two P-metric sizes and carries that diameter ratio into circumference, axle-center clearance and theoretical speedometer error.
How to use this calculator
Enter width, aspect ratio and rim diameter for the original and proposed tire.
Enter any indicated speed as a reference point.
Compare diameter and circumference change, theoretical actual speed and axle-center clearance change.
Where people use it
- •Checking a wheel/tire size change
- •Estimating speedometer difference after upsizing
- •Comparing sidewall and overall diameter before buying tires
Example: 265/70R17 to 285/70R17
The wider 285 tire also has a taller sidewall at the same 70% aspect ratio, increasing overall diameter and making actual road speed slightly higher than an unchanged speedometer indication.
What the result does not assume
- •This is geometry, not a vehicle fitment approval. Check wheel width, load/speed ratings, suspension/body clearance and manufacturer guidance separately.
- •Real rolling circumference changes with pressure, load, tread and tire construction, so GPS/measured speed can differ from the geometric estimate.
Frequently asked questions
Why does aspect ratio affect tire height?+
Aspect ratio is sidewall height as a percentage of tire width, so width and aspect ratio both affect sidewall height.
Does a larger tire make the speedometer read low?+
With calibration unchanged, a larger rolling diameter generally travels farther per revolution, so theoretical actual speed is higher than the indicated speed.
Is a 3% diameter change always safe?+
No universal percentage proves fitment or safety. Vehicle systems, clearance and manufacturer requirements vary.
Semantic next steps
Continue the calculation
These links move to a different input, formula or project stage rather than a keyword variation of this page.
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