Enter your current and new tire sizes to compare overall diameter, circumference, revolutions per mile, and speedometer error before you buy new tires.
Your results will appear here.
Enter your values and press Calculate.
A tire size like 225/45R17 packs three measurements into one code: section width in millimeters, aspect ratio as a percentage of that width, and rim diameter in inches. This calculator turns those numbers into the tire's true overall diameter by converting the sidewall to inches and adding two sidewalls to the rim. From the diameter it derives the circumference, the revolutions per mile, and the percentage change between your old and new tires. Because speedometer and odometer readings scale directly with diameter, the same percentage tells you exactly how far off your speedometer will read.
Sidewall height = Width × (Aspect / 100). For a 225/45 tire, that is 225 × 0.45 = 101.25 mm. Overall diameter = Rim + 2 × Sidewall / 25.4, converting millimeters to inches. So 225/45R17 gives 17 + 2 × 101.25 / 25.4 = 24.97 inches. Circumference = pi × diameter, and revolutions per mile = 63360 / circumference, because 63,360 is the number of inches in a mile. The diameter change percentage between old and new tires is (New - Old) / Old × 100, and your speedometer error is the same percentage. This comparison shows both tires side by side so the change is obvious.
Take 205/55R16. The 205 is the section width in millimeters, measured across the tread. The 55 is the aspect ratio: the sidewall height is 55% of the width, or 112.75 mm. The R means radial construction, which nearly every modern tire uses. The 16 is the wheel rim diameter in inches. Some sizes carry extra letters up front, like P for passenger or LT for light truck, which describe the intended vehicle rather than the dimensions. In the UK this tool is often called a tyre size calculator, and the math is identical.
Old tire 205/55R16: sidewall = 205 × 0.55 = 112.75 mm = 4.44 in; diameter = 16 + 2 × 4.44 = 24.88 in; circumference = 3.1416 × 24.88 = 78.2 in; revs per mile = 63360 / 78.2 = 811. New tire 225/45R17: sidewall = 225 × 0.45 = 101.25 mm = 3.99 in; diameter = 17 + 2 × 3.99 = 24.97 in; circumference = 78.5 in; revs per mile = 808. Diameter change = (24.97 - 24.88) / 24.88 × 100 = +0.4%. At a speedometer reading of 60 mph, your actual speed is 60 × (24.97 / 24.88) = 60.23 mph, a negligible difference.
Your speedometer and odometer are calibrated to the factory tire diameter. A taller new tire covers more ground per revolution, so the speedometer reads lower than your true speed; a shorter tire does the opposite. The error is exactly the diameter change percentage: with +0.4% tires, a 60 mph reading means 60.23 mph actual, and your odometer undercounts miles by the same 0.4%. The widely used 3% rule says to keep the new diameter within plus or minus 3% of the original, because beyond that the error starts to matter for speeding tickets, odometer accuracy, and vehicle electronics like ABS and traction control.
Plus-sizing is the art of fitting a larger rim while keeping the diameter close to stock: you go up an inch in rim size and down in aspect ratio so the overall height barely changes. For any tire upgrade, always check physical clearance, since width and sidewall changes can rub the wheel well or suspension even when the diameter is fine. Keep the diameter within 3% of the original to avoid confusing your ABS, traction control, and transmission shift points. A tire size chart from the manufacturer lists the exact mounted diameter for each size. Remember that real mounted tires vary by brand, tread depth, and inflation pressure, so treat these figures as nominal and confirm with the manufacturer's spec sheet before buying.
Convert the sidewall to inches with Width × Aspect / 100 / 25.4, then add two sidewalls to the rim: Diameter = Rim + 2 × (Width × Aspect / 100) / 25.4. For 205/55R16 that gives 16 + 2 × (205 × 0.55 / 25.4) = 24.88 inches.
A taller tire travels farther per revolution, so your speedometer reads lower than your actual speed. A 3% taller tire shows 60 mph when you are really doing about 61.8 mph. A shorter tire has the opposite effect: the speedometer reads higher than reality.
The 3% rule is the industry guideline that a replacement tire should stay within plus or minus 3% of the original overall diameter. Inside that band, speedometer error stays negligible and vehicle electronics like ABS and traction control keep working normally. Larger changes can throw off the odometer, shift points, and safety systems.
Revolutions per mile is how many times the tire turns over one mile of road: 63360 divided by the circumference in inches. It is the number that links tire size to your speedometer and odometer. Comparing the old and new figures shows exactly how much your readings will drift after a tire change.
Not by itself. Only the overall diameter affects the speedometer. A wider tire with a lower aspect ratio can have nearly the same diameter as the original, which is the whole point of plus-sizing. Always compare diameters, not widths, when judging speedometer impact.
Inside the 3% rule, recalibration is unnecessary because the error is tiny. For bigger changes, many vehicles can be reprogrammed by a dealer or with an aftermarket tuner so the speedometer reads correctly again. Until then, your actual speed is the indicated speed multiplied by the new-to-old diameter ratio.
Ground clearance under the axles rises by half the diameter increase, because only the bottom half of the extra height lifts the car. A tire that is 1 inch taller overall adds about 0.5 inches of clearance. Lift and suspension clearance are separate and do not change with the tire.
Your car's correct sizes are listed on the sticker inside the driver's door frame and in the owner's manual. Enter one of those as the current size in this calculator, then compare any size you are considering. Stay within the 3% rule and confirm rim width compatibility before buying.