QUOTE (Grawburg)". . . I have a 2004 XC90 with P235/16 R18 tires . . ."
You probably mean 235/60/ R18.

The 235 is the width in MM of the tire track on the road, and the next number (60 or 65) is the ratio, or percentage of that width (235 MM) which represented in the profile, or the distance between the road and the wheel . . . . the air cushion of the tire.

At any rate, your mechanic is correct, you cannot put 17" Tires on 18" Rims.

Maybe he can find a Buyer for these tires that you cannot use ?

Just goes to show that being off by only 1 digit can make for big trouble. Numbers are seldom forgiving.

Check your Owner's Manual for the correct tire size . . . . or see i it's on the inside of your Fuel Cover. Depending upon your Model, it will probably read 235/65/R17 for 17" wheels, and 235/60/R18 for your current 18" wheels.

Maybe you can find an appropriately sized set of used 17" wheels to go with the new 17" tires that will otherwise go unused ?

 

The "R" you are referring to stands for radial construction. Almost everything is now of days except for trailer tires due to bias ply construction being able to handle heavier loads. That and some off road applications where the bias plys handle rough terrain better.

Back to YOUR problem. What he said.

The other thing you might be able to do is barter with someone to order a new set of 18's in your cars size and you give them the 17's and some cash.

I'm sorry to say it's either smaller wheels to fit the tires or bigger tires to fit your wheels.