I have a 2002 Mitsubishi Galant ES, it has the stock 16" rims and I would like to replace it with an 18" one. My question would be, do I need to change the coil springs to accommodate the bigger 18" wheel size?
The best place to look for allowable wheel and tire combinations is your owner’s manual. Anything that alters the suspension alters vehicle stability. Anything that alters total tire circumference would render the odometer inaccurate and would likely cause interference between the tires and the wheel wells. That would make the car unsteeerable and unsellable.
Usually you just use tires that are tiny and hard. That keeps the circumference about the same.