Plus sizing:
bigger wheel, same tyre height
The whole trick is a swap. Every inch you add to the rim you take off the sidewall, and the rolling diameter — the only dimension the car's electronics actually care about — never moves.
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·both tyres drawn to one scale from their size designations; the ±3% ring is the industry fitment convention, not a legal limit·REVIEWED ETRTO STANDARD
±3% The target is to keep the new rolling diameter within 3% of the original. That is the band vehicle makers calibrate the speedometer, ABS and stability control against — it is a convention, not a law, and it is the reason plus sizing works at all.
An inch on the rim is 25.4 mm of diameter, which means 12.7 mm off each sidewall to break even. On a 205 mm tyre that is roughly six points of aspect ratio per inch.
A worked chain, from 205/55R16
Each step goes up one rim inch and down on the sidewall to hold the diameter. Watch the last column stay near zero — that is the whole exercise.
| Step | Size | Diameter | Sidewall | Change vs original |
|---|---|---|---|---|
| Original | 205/55R16 | 631.9 mm | 112.8 mm | — |
| Plus 1 | 215/45R17 | 625.3 mm | 96.8 mm | −1.04% |
| Plus 2 | 225/40R18 | 637.2 mm | 90.0 mm | +0.84% |
| Plus 3 | 245/35R19 | 654.1 mm | 85.8 mm | +3.51% |
SOURCE·Metric size geometry + UN ECE R39 speedometer tolerance·a worked example, computed from the four designations·REVIEWED ETRTO STANDARD
The rule of thumb, and where it breaks
Up one rim inch, down roughly ten points of aspect ratio and up one width step is the traditional recipe. It works because those three changes very nearly cancel — but "very nearly" is doing real work in that sentence, and the only way to know is to compute it.
Where it breaks: the aspect ratios come in five-point steps and the widths in ten-millimetre steps, so past plus-two there is often no combination that lands inside ±3%. That is the point at which plus sizing stops being a swap and starts being a modification, with a speedometer error attached.
What the shorter sidewall costs
- Ride. The sidewall is the only real suspension component between the rim and the road. Halve it and the dampers inherit work they were not tuned for.
- Pothole risk. Less rubber between the rim edge and the impact. This is the single most common way a plus-two fitment gets expensive.
- Unsprung mass. A bigger wheel is usually a heavier wheel, and unsprung mass is the worst kind to add.
- Cost. Low-profile sizes in wide fittings are a smaller market and priced like one — often twice the money for the same brand and pattern.
- Clearance. A wider tyre can foul at full lock even when the diameter is perfect, and the diameter being perfect is what tempts people to skip checking.
Plus sizing — the questions that follow
What is plus sizing?
Fitting a larger-diameter wheel with a lower-profile tyre so that the overall rolling diameter stays the same. Plus one is one inch up on the rim with a correspondingly shorter sidewall; plus two is two inches. The point is to change the look and the steering response without touching the speedometer.
Does plus sizing affect my speedometer?
Done properly, almost not at all — that is the definition of doing it properly. The target is to stay within ±3% of the original rolling diameter, at which point the speedometer, ABS and stability control all remain inside the band they were calibrated for.
What are the downsides of plus sizing?
A shorter sidewall is a stiffer sidewall: sharper turn-in, more road noise, a harsher ride and considerably more exposure to pothole damage on both tyre and rim. Wider tyres also generally cost more and can rub at full lock. Nothing here is free.
Work your own chain on the calculator. The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.