How to Use the Tire Size Change Fitment Calculator
Switching to bigger wheels or a different tire size changes your overall tire diameter, which can throw off your speedometer, cause suspension rubbing, and affect fuel economy. This calculator takes your current and new tire width, aspect ratio, and rim size, then calculates the diameter change so you can judge whether the new size fits safely.
Overall tire diameter is rim size (converted to mm) plus sidewall height doubled. Increasing width while lowering the aspect ratio (for example, going from 205/55R16 to 215/50R17) shortens the sidewall and can offset most of the diameter change — this is commonly called "plus sizing." Widening the tire while keeping the same aspect ratio, on the other hand, tends to increase overall diameter and throw off your fitment.
A larger-than-stock diameter makes your speedometer read lower than your true speed (which can feel like a bonus but actually risks unintentional speeding), while a smaller diameter makes it read higher than actual. Staying within ±3% of stock is generally fine, but going past ±5% raises the risk of fender rubbing, suspension damage, and ABS or cruise control glitches — so run the numbers and check with a shop before committing to a size change.
Frequently Asked Questions
A larger overall diameter makes your speedometer read lower than your actual speed and can cause fender rubbing, while a smaller diameter makes it read higher than actual. Bigger speedometer and ABS errors can also affect safety.
Staying within ±3% of your factory diameter is generally recommended, and you shouldn't exceed ±5%. Beyond that, suspension rubbing, speedometer error, and reduced fuel economy become much more noticeable.