Plus-sizing 225/40R18 to 21″ wheels on 295 mm tires

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 3″ 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 21″ wheel with a 295 mm tire needs an aspect ratio of 17.6. Tires are not made in continuous aspect ratios — the nearest is 25, which lands the new diameter at 26.81″, or 6.86% 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 2.9″. 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

295 mm is the widest point of the sidewall. Check it at full steering lock and full suspension compression, not sitting still in the driveway.