📋 The Core Rule
The cardinal rule of plus sizing: keep overall tire diameter within 3% of stock. Beyond 3%, speedometer error, ABS slip detection, traction control, and transmission shift points all start to drift. Most TPMS systems also calibrate around the OEM rolling circumference.
The wheel grows by 1, 2, or 3 inches, and the tire sidewall shrinks proportionally to compensate.
🔹 Plus 1, Plus 2, Plus 3
| Level | Wheel | Sidewall | Tradeoff |
|---|
| Stock | 17" | 215/55R17 | baseline |
| Plus 1 | 18" | 225/45R18 | slightly firmer ride |
| Plus 2 | 19" | 235/40R19 | firmer ride, sharper steering |
| Plus 3 | 20" | 245/35R20 | harsh ride, curb-rash risk |
📐 The Math
Overall diameter = wheel diameter + (2 × sidewall height in inches). Sidewall height (mm) = section width × aspect ratio. Convert mm to inches by dividing by 25.4.
For 215/55R17: sidewall = 215 × 0.55 = 118.25mm = 4.66 in. Diameter = 17 + (2 × 4.66) = 26.31 in.
For 225/45R18 (Plus 1): sidewall = 225 × 0.45 = 101.25mm = 3.99 in. Diameter = 18 + (2 × 3.99) = 25.97 in. Difference: 1.3%. Within spec.
⚖️ Pros and Cons
Pros: sharper steering response, more sidewall stiffness, room for bigger brakes, better looks, wider contact patch options.
Cons: harsher ride, more pothole damage, heavier wheel-tire combo can hurt acceleration and MPG, narrower tire selection at extreme plus sizes.
⚠ Load index mattersPlus-sized tires must meet or exceed the OEM load index. Many ultra-low profile tires have lower load indexes, which is a problem on SUVs and trucks.
❓ Frequently Asked Questions
What does Plus 1 mean for tires?
Plus 1 means you go up one inch in wheel diameter (e.g., 17" to 18") and reduce sidewall height to keep overall tire diameter the same. Speedometer accuracy stays intact.
Can plus sizing void my warranty?
Going within manufacturer-approved sizes (often listed on the door jamb or in the owner's manual) does not void the warranty. Going to extreme sizes can affect suspension and drivetrain warranty claims.
Does plus sizing affect MPG?
Yes, slightly. Heavier wheel-tire combos increase rolling resistance and rotating mass, typically costing 1–3% MPG depending on how aggressive you go.
How do I know if my plus size fits?
Calculate the overall diameter and confirm it is within 3% of stock. Also verify load index, speed rating, fender clearance, and that the wheel's offset and bolt pattern match.
Is plus sizing safe?
Yes, when done within reasonable diameter, load, and offset tolerances. It is widely accepted and even offered by automakers as factory packages.
What is the maximum plus size?
Most passenger cars top out at Plus 3 (e.g., 17" to 20"). Beyond that, sidewall becomes so thin that pothole damage is virtually guaranteed.
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