Vehicle · Tires
Tire plus sizing calculator.
Find tire sizes for a larger rim that keep the overall diameter close to your current tire.
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What is plus sizing a tire?
Plus sizing means fitting a larger-diameter wheel and a lower-profile tire so the overall diameter, and with it the speedometer, odometer and gearing, stay close to the original. The geometry is simple. Overall diameter = rim diameter x 25.4 + 2 x (section width x aspect ratio / 100) in millimetres. For a new rim, the sidewall that keeps the old diameter is (old diameter - new rim x 25.4) / 2, and dividing by the tire width gives the aspect ratio, which this tool rounds to the nearest 5 because sizes are sold in those steps. 49 CFR 571.109 S5.1(f) determines tire outer diameter from the maximum circumference divided by pi, which is the same quantity.
A commonly quoted rule of thumb says to stay within about 3% of the original overall diameter. That is a guideline repeated by tire and wheel retailers, not a published regulation, and your vehicle maker may allow less. A larger tire makes the speedometer read lower than your true speed by about the same percentage, and wheel-speed based systems such as ABS expect the original size. Example: 205/55R16 is 631.9 mm tall and 225/45R17 is 634.3 mm, a +0.4% difference.
Frequently asked questions
What tire replaces 205/55R16 on a 17 inch rim?
A 225/45R17 is about 0.4% taller than 205/55R16 (634.3 mm vs 631.9 mm overall), so the speedometer would read about 0.4% lower than true speed. Confirm the wheel width, load index and speed rating with a tire shop.
What is the 3% rule for tires?
A rule of thumb that the new overall diameter should be within about 3% of the original. It is not a regulation and some makers want less, so check your owner's manual or tire placard.
Does plus sizing change my speedometer?
If the new tire is taller than the old one, the true speed is higher than the speedometer shows by about the same percentage. A shorter tire does the reverse.
Can I make the tire wider as well?
The list shows widths from 20 mm narrower to 40 mm wider. A wider tire needs a suitable rim width and clearance, which this tool does not check.
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Scope
Diameter geometry only. It does not check rim width, load index, speed rating, brake clearance or your vehicle maker's approved sizes. Use the sizes on the vehicle placard as the reference.