Plus sizing is a larger wheel with a lower-aspect tire, picked so overall diameter stays near the stock size. Plus one is usually one inch more rim. The ride gets firmer. The speedometer stays honest if you did the arithmetic. Most people skip the arithmetic. The calculator is that check.
A worked plus-one
Stock 265/70R17 is 31.6 in overall. Plus one on an 18-inch rim wants a 265/65R18 or a 255/70R18. 265/65R18 is 31.6 in. 255/70R18 is 32.1 in. The first is the plus-size. The second is a slight upsize wearing a plus-size costume.
| Size | Rim | Diameter | vs stock |
|---|---|---|---|
| 265/70R17 | 17 in | 31.6 in | stock |
| 265/65R18 | 18 in | 31.6 in | 0.0% |
| 255/70R18 | 18 in | 32.1 in | +1.4% |
What you gain and what you give up
- Steering. A shorter sidewall flexes less. Turn-in feels sharper. That is the point on a sedan.
- Ride. The same shorter sidewall is less air over a pothole. You feel more of the road because there is less tire between you and it.
- Wheel cost. Plus sizing is often an excuse to buy wheels. Price that in.
- Curb damage. Less sidewall is less protection for the rim.
Plus sizing is the wrong tool for an off-road upsize. If you want more diameter, you want more diameter, not a bigger wheel. Use the comparison and say so.
If the plus-size lands more than about 3% off stock diameter, it is not a plus-size. It is a different tire. Recalibrate or pick again.