My 350 z is acting like tranny is sliping
 

Is there a problem using different tires size on rear of my 350z than what specs are

some mechanic said that's making my car say sliping on the dash and it not going in gear please any help with this is very welcomed

Short answer: Yes and no.

The yes part: What's your trim level? Sounds like you're having VDC or TCS issues due to a tire too short in the back. What size?

And "no" because none of those issues should keep your trans from engaging.

245 45 zr 17 94w
 

235 45 zr 17

235 not 245
 

235 not 245

235 not 245
 

235 not 245

You OK br0? Wake-and -Bake often?

Heh heh....

But back to the original question.... yes, 235/45 is almost an inch shorter 0.93") than the stock 235/50-17. Since diameter is smaller, the wheelspeed is higher given any speed the vehicle is traveling. This looks - to the ECM - like the tire is spinning and initiates VDC or TCS to compensate, throwing the SLIP light. Get the right size (height) tire on there and the problem goes away.