The rule: same height, bigger wheel
Your speedometer, odometer, ABS and traction control are all calibrated to the original overall diameter of the wheel and tire. Keep the new package within about 3% of that, and every one of them stays accurate.
Since the wheel is getting bigger, the only place to give up height is the sidewall. A 17-inch wheel with a tall sidewall and a 20-inch wheel with a short one can be almost exactly the same height.
A common example: a truck on 265/70R17 tires is about 31.6 inches tall. A 265/55R20 is about 31.5 inches: the same width, the same height, three more inches of wheel.
Plus one, plus two, plus three
Plus one means one inch more wheel, plus two means two, and so on. Each step takes roughly the same amount out of the sidewall height.
Plus one is almost always comfortable. Plus two is the popular sweet spot. Plus three and beyond is where the sidewall gets short enough that you notice it in the ride and in pothole season.
What changes when the sidewall shrinks
Ride: a shorter sidewall flexes less, so bumps come through more directly. On a truck with a soft suspension it's a mild change; on a car already on low-profile tires it can be harsh.
Steering: less sidewall flex means sharper turn-in and less body roll in quick lane changes. This is the real performance gain of plus sizing.
Protection: a tall sidewall cushions the rim against potholes and curbs. A short one leaves the rim more exposed to bends and scrapes.
Cost: larger-diameter tires usually cost more and there are fewer models to choose from, especially in all-terrain and winter tires.
Weight: bigger wheels are often heavier. More weight in the wheels can slightly dull acceleration and braking and cost a little fuel economy.
Width: go a little wider, not a lot
Staying at the same tire width is the safest choice. Going one or two sizes wider (10 to 20 mm) fills the arch and adds grip, but check the offset: a wider tire on a wider wheel changes where the edges sit.
The tire also has to suit the wheel width. Every tire size has an approved range of rim widths, and the retailer's fitment check covers this.
Truck owners: if you tow or haul, match or exceed the original load index. A lower load rating on a bigger wheel is a real safety problem, not a cosmetic one.
Questions
Do I need to recalibrate my speedometer after plus sizing?
Not if the new package is within about 3% of the original height. Beyond that, the speedometer will read off, and some vehicles also need the ABS and stability system recalibrated.
Will bigger wheels make my car faster?
No. Bigger wheels are usually heavier, which slightly slows acceleration. What improves is steering response, because the shorter sidewall flexes less.
Is plus sizing bad for winter?
Taller sidewalls and narrower tires work better in snow. Many drivers keep the bigger wheels for the warm months and a smaller set with winter tires for the cold ones.