i am looking for the Kumho Ecsta MX front use 225/40R18 220-AA-A Y 88 BW

and rear use 245/40R18 220-AA-A Y 93 BW.

this is the shop said:
the 225/40/18 and 245/40/18 sizes are quite a bit too short for your 350z. The vehicle normally takes a 225/45/18 and 245/45/18. Going with a smaller overall diameter will change the speedometer readings and other instrumentation, as well as altering your tranny gearing and abs, which is not recommended. Did you want us to find you tires in the proper sizing for your car.

And they said i only can use W speed ratio. i can't use Y speed ratio.

now i am using falken rt615 use different spec between the factory, but i didn't feel any different.

 

The recommended speed rating for the 350Z is W. So you can use a W or better rated tire:

W
Y