Tires: Tire size for cx9

Question
I have cx9 Mazda I'm not driving fast and it's only like 12 km a day
I would like to change the tire from the size 245 50 r20 to 285 40 r20 due that the first one not available or if it ms available the price too high as well as my friend will give me this size for free because its available in his shop :)
My question did it will effect too much to my car ??
Thanks for your support

Answer
Ahmed,

First, the wheels are too narrow.  The allowable rim width for a 285/40R20 is 9 1/2 to 11, and your stock wheels are 8 1/2".  Using a rim that is too narrow results in the tires wearing in the center, and directional instability, especially in cross winds. In other words, the car will wander.

Second, even if you use the proper wheel size, it is hard to tell what affect this change will have as the difference between the 2 makes and models are going to have a HUGE effect.  But assuming EVERYTHING else is the same, directionally, this to going towards a harsher ride, less fuel economy, and better handling.