Tire Size Calculator
Compare any two tire sizes side by side. Enter your current size and a new one to see the difference in overall diameter, sidewall height, and revolutions per mile — plus the speedometer error and whether the change stays within the safe ±3% range.
How to read the results
Tire sizes like 265/70R17 don't tell you the overall height directly — a 265/70R17 and a 285/65R18 are nearly the same diameter despite looking different on paper. This calculator converts both sizes into their real physical dimensions so you can compare what actually matters: how tall each tire is, how far it rolls per turn, and how much a change will throw off your instruments.
The ±3% rule
The widely used guideline is to keep a new tire within about ±3% of the original overall diameter. Stay inside that window and your speedometer, odometer, ABS, traction control, and transmission behavior remain close to how the vehicle was engineered. Go beyond it and the errors grow, towing and payload numbers shift, and on many vehicles you'll eventually need a speedometer recalibration — and, for big jumps, a lift or re-gearing.
Speedometer & odometer effect
Your speedometer counts wheel revolutions and assumes the factory tire diameter. A larger tire covers more ground per revolution, so the needle reads slower than your true speed (and your odometer under-counts miles). A smaller tire does the reverse. The calculator shows your true speed at any indicated speed, so you can see the real-world effect before you buy.
Diameter vs. clearance
Diameter tells you whether your speedometer and gearing will be happy — it does not tell you whether the tire physically fits. Width and wheel offset decide rubbing, and clearance has to be checked at full steering lock and full suspension compression. Use this tool for the dimensional math, then confirm fitment against your vehicle's door placard and a model-specific guide.
Frequently asked questions
How much bigger can I go without affecting my speedometer?
As a rule of thumb, keep the new tire within about ±3% of the original overall diameter. Beyond that your speedometer and odometer read noticeably off, and your ABS, traction control, and transmission shift points were all calibrated to the original size. The calculator shows the exact percentage difference and the resulting speedometer error.
How does a bigger tire change my speedometer reading?
A larger-diameter tire travels farther per revolution, so the speedometer — which counts revolutions — reads slower than you are actually going. A tire 3% larger means that at an indicated 60 mph you are really doing about 61.8 mph. A smaller tire does the opposite, showing a higher speed than real.
What is "plus sizing"?
Plus sizing fits a larger-diameter wheel with a lower-profile tire so the overall diameter stays the same. Each inch of wheel diameter is offset by a lower aspect ratio, keeping the rolling diameter — and your speedometer — unchanged while sharpening handling and changing the look.
Will a bigger tire actually fit my vehicle?
Diameter is only part of it — width and wheel offset decide whether a tire rubs. This calculator gives you the exact dimensional difference; for real clearance, check at full steering lock and full suspension compression, and consult a model-specific fitment guide.