Plus-sizing 225/45R17 to 21″ wheels on 255 mm tires
| Sidewall height available(24.97″ diameter − 21″ rim) ÷ 2 | 1.99in |
|---|---|
| Aspect ratio that would needbelow anything made — the rim is too big for this diameter | 19.8 |
- Current size
- 225/45R17
- New wheel
- 21 in
- New width
- 255 mm
The whole point is that diameter does not change
Plus-sizing means a bigger wheel and a shorter sidewall, adding up to the same rolling diameter. Go up 4″ of rim and the sidewall has to lose exactly half of that on each side.
Holding 225/45R17's 24.97″ overall diameter on a 21″ wheel with a 255 mm tire needs an aspect ratio of 19.8. Tires are not made in continuous aspect ratios — the nearest is 25, which lands the new diameter at 26.02″, or 4.19% from stock.
The target here is ±1%, not ±3%
The general limit for tire changes is 3%, because that is where speedometers, ABS and stability control start reading meaningfully wrong. Plus-sizing is held to about 1%, and the reason is simple: this is a change you chose to make. There is no reason to accept an error you introduced for looks.
What you are trading away
Sidewall goes from 3.99″ to 2.51″. That is sharper steering response and a firmer ride, and it is also a much higher chance of bending a wheel on a pothole. Below about 4 inches of sidewall, potholes start costing wheels rather than tires.
The wheel also gets heavier as the tire gets lighter — and the wheel is unsprung, rotating mass, which is the worst possible place to add weight for acceleration, braking and ride. Check the actual wheel weight before assuming bigger looks better and drives better.
Section width is not tread width
255 mm is the widest point of the sidewall. Check it at full steering lock and full suspension compression, not sitting still in the driveway.
Notes
- This combination does not exist. Holding 225/45R17's 24.97″ overall diameter on a 21″ rim leaves 1.99″ of sidewall, which on a 255 mm tyre is an aspect ratio of 19.8 — outside the 25–75 range anything is made in.
- Either step to a smaller rim, or change the section width. Wider tyres need a lower aspect ratio for the same sidewall height, so going wider is usually what makes a large rim step work.
Nearby sizes
- Plus-sizing 215/45R17 to 21″ wheels on 255 mm tires
- Plus-sizing 225/65R17 to 21″ wheels on 255 mm tires
- Plus-sizing 205/65R15 to 21″ wheels on 255 mm tires
- Plus-sizing 235/45R17 to 21″ wheels on 255 mm tires
- Plus-sizing 225/45R17 to 20″ wheels on 255 mm tires
- Plus-sizing 225/45R17 to 22″ wheels on 255 mm tires
- Plus-sizing 225/45R17 to 19″ wheels on 255 mm tires
- Plus-sizing 225/45R17 to 21″ wheels on 245 mm tires
- Plus-sizing 225/45R17 to 21″ wheels on 265 mm tires
- Plus-sizing 225/45R17 to 21″ wheels on 235 mm tires
- Plus-sizing 225/45R17 to 21″ wheels on 275 mm tires
Read more
- What Is a 33-Inch Tire in Metric? Seven Answers, RankedSeven common metric sizes get called 33s. They span half an inch in height and nearly two inches in width, and the width is what decides whether they fit.
- Reading a Tire Sidewall: Every Marking, in OrderThe size is the part everyone reads. The characters after it are the part that decides whether the tire is legal on your vehicle.
- Metric to Inches Tire Size: Both Directions, WorkedMetric to inches is arithmetic with one answer. Inches to metric has many answers, because you are choosing among sizes that happen to land near your target.
- Comparing Two Tire Sizes: The Four Numbers That ChangeComparing sizes by diameter alone hides the change that most often causes the problem. Four numbers move when you change a tire, and they move independently.