Plus-sizing 225/40R18 to 20″ wheels on 225 mm tires
| Recommended size | 225/30R20 |
|---|---|
| Exact aspect ratio needednearest made is 30 | 28.7 |
| Overall diameter+0.91% | 25.09″ → 25.31″ |
| Within ±1% (plus-size target)as close as the available sizes allow | Yes |
| Within ±3% (general limit)speedometer, ABS and stability control all read off this | Yes |
| Sidewall height-25% — this is what changes the ride | 3.54″ → 2.66″ |
| Section width0 mm — check clearance at full lock | 225 → 225 mm |
| Rim stepplus 2 | 18″ → 20″ |
Notes
- Plus-sizing keeps the overall diameter and changes everything else. Going up 2″ of rim means losing 0.89″ of sidewall on each side, which is why the aspect ratio has to come down from 40 to 30.
- 0.91% is inside the ±1% that plus-sizing aims for. This is a discretionary change, so the standard is tighter than the ±3% used for off-road tyre upgrades — there is no reason to accept an error you chose to introduce.
- Less sidewall is the trade you are making, and it is a real one. 2.66″ of rubber between the rim and the road means sharper steering response, a firmer ride, and a much higher chance of bending a wheel on a pothole. Below about 4″ of sidewall, potholes start costing wheels rather than tyres.
- The wheel gets heavier as the tyre gets lighter, and the wheel is unsprung and rotating. Plus-sizing frequently adds weight where it hurts acceleration, braking and ride quality most — check the actual wheel weight before assuming bigger looks better and drives better.
- Section width is the widest point of the sidewall, not the tread. 225 mm needs checking at full steering lock and full suspension compression, not just sitting still.
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 2″ of rim and the sidewall has to lose exactly half of that on each side.
Holding 225/40R18's 25.09″ overall diameter on a 20″ wheel with a 225 mm tire needs an aspect ratio of 28.7. Tires are not made in continuous aspect ratios — the nearest is 30, which lands the new diameter at 25.31″, or 0.91% 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 3.54″ to 2.66″. 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
225 mm is the widest point of the sidewall. Check it at full steering lock and full suspension compression, not sitting still in the driveway.
Nearby sizes
- Plus-sizing 245/65R17 to 20″ wheels on 225 mm tires
- Plus-sizing 235/40R18 to 20″ wheels on 225 mm tires
- Plus-sizing 245/45R17 to 20″ wheels on 225 mm tires
- Plus-sizing 235/60R18 to 20″ wheels on 225 mm tires
- Plus-sizing 225/40R18 to 19″ wheels on 225 mm tires
- Plus-sizing 225/40R18 to 21″ wheels on 225 mm tires
- Plus-sizing 225/40R18 to 18″ wheels on 225 mm tires
- Plus-sizing 225/40R18 to 22″ wheels on 225 mm tires
Read more
- Load Index and Speed Rating — The Two Codes at the End of the SizeEveryone reads the size and stops. The two characters after it are the ones with a safety consequence.
- Speedometer Error From Tire Size — One Ratio, and It Runs Both WaysA one-inch increase in diameter on a 30-inch tire is a 3.3% error. At an indicated 70 you are doing a bit over 72.
- Recalibrating a Speedometer After a Tire Change: Four Routes, and How to Verify It WorkedOn most modern vehicles the tire size is a stored number, not a mechanical ratio. Where that is true, correcting it takes minutes; where it is not, the options get physical.
- Plus Sizing Chart: Wider Tire, Shorter Sidewall, Same Rolling DiameterPlus sizing trades sidewall for wheel and keeps the overall diameter where it was. Done properly nothing on the vehicle notices — which is the entire point.