Reducing tire size from 19” to 18” to improve ride 2015 RX

[Jolanza]

I have read on line where some others have done this. They bought the 18” rims and new tires. Worked great.

[Les Lex 2018]

You have to take into account the fact that you are putting more flexible rubber between the road and your wheel, which changes the characteristics of an emergency maneuver, like jerking the steering wheel to avoid an object in the roadway, usually at night.

While you could argue that the difference is unnoticeable, the science says that it will make some kind of difference (in inches most likely), and that increased distance of forward vs. lateral movement might be just enough to allow you to hit what ever it is you don't want pasted on your expensive RX grill.

The larger size rims just naturally look better and allow for making Jack Rabbit lane changes just a "little" bit quicker, based on my perception anyway.  

[RX400h]

I believe your model year was available with either 235/60/18 or 235/55-19.

My guess is that the model with 19-inch tires handles a bit better but its wheels are heavier, so braking distances may be a bit longer and the ride, a bit harsher. So yes, there is a tradeoff. You can see the OEM wheel sizes and how the 18 inchers look on your vehicle at www.tirerack.com.