The idea in one sentence
Plus sizing means going up an inch in rim diameter and taking an inch out of the sidewall, so the tire's overall diameter stays about the same. Minus sizing is the same trade in reverse, usually for a winter fitment where sidewall is wanted and wheel diameter is not.
Holding the overall diameter is what keeps the speedometer, the odometer and any system that counts wheel revolutions reading the same as before. That is the whole point of doing it by arithmetic rather than by eye.
Working it out
Compute the original overall diameter, then look for sizes on the target rim whose overall diameter is close to it. Every size page on this site lists the closest plus one and minus one candidates already, ranked by how little the diameter moves, and the comparison tool will do it for any pair.
A three percent window is the conventional threshold for calling a swap like for like. That figure is a widely repeated trade convention rather than a clause of any standard, and this site labels it as one wherever it appears.
What changes even when the diameter does not
A shorter sidewall is a stiffer sidewall. Steering response usually sharpens, ride quality usually suffers, and the tire has less rubber between the rim and a pothole. A wider tread changes the contact patch shape, which affects wet behaviour and tramlining, and a wider tire on the same rim width sits differently from one on a rim matched to it.
Load capacity is not automatic either. A different size is a different load index, and the replacement has to meet or exceed what the vehicle placard specifies. A size that is perfect on diameter and short on load index is not a candidate.
The four things arithmetic cannot tell you
This site ranks numbers, and there are constraints that are not numbers it holds.
- Clearance. Whether the tire clears the strut, the liner and the arch at full steering lock and full suspension travel is a physical question about a specific vehicle.
- Wheel fitment. Rim width and offset have to suit both the tire and the vehicle, and a size that fits the tire may not exist in a wheel that fits the car.
- Availability. A size can be arithmetically ideal and made by nobody in the load index the vehicle needs.
- The placard. Load index, speed rating and inflation pressure come from the vehicle, not from the tire catalog.