Hi, I’ve got an 08 Rx 350. It came with a set of 17” winter tires and the summer set are 18”. I assume the spare is 18”.
If I have a flat while running the 17” set I assume I can’t use the 18” spare, is this correct?

 

So long as the rolling diameter is the same it should be ok, but it’s not ideal.

 

Thanks 02SDGS, I’m thinking I had better get a 17” spare.

 

This discussion brings up a good point. I'm planning on buying an extra set of wheels so that I have a set for winter tires and a set for summer tires. My '09 RX 350 came with 18" rims, and now that you point out the spare tire challenge of having two different sizes, I I shouldn't buy 17" wheels (unless the rolling diameter is the same). My plan has been to pick up some nice looking wheels for the summer set, and use the OEM rims for the winter tires.

 

rolling diameter is normally the same . Different profile tires go on different size rims. If it is different the speedo will be out.

 

+1, 08 RX350 had 17 inch rims standard, but 18 inch optional, so I expect both to have same rollout.

 

You can use this calculator to figure out the various diameters.

https://www.wheel-size.com/calc/

 

Well, I looked into this quite deep actually.
So, here's the wisdom.
We do not have mechanical transfer case, like a true 4x4 vehicle. We have what is called a viscous differential between front and rear axles.
In case of true mechanical transfer case, ALL wheels MUST be same rolling diameter or, transfer case wil lock up in very short time. I actually experienced that once and that's how I did my research.
With AWD models, you can get away with mismatched wheels, but not permanently. So yes, you can get out of trouble to safety on mismatches but, you should restore all 4 same as soon as possible.

 

Lexus has 2 factory tire and rim sizes for this vehicle that are equal. 17" rims with 225/65/17 and 18" rims with 235/55/18. Both are equally 28" diameter. The 17 inch tire has a taller sidewall with the ratio of 225/65. 18" tire shorter sidewall with its ratio of 235/55.
17 inch tire sidewall will be taller, 18' tire will be wider width. Both equal in height and diameter. I prefer a taller, skinnier snow tire myself. They have a squishier feel that grabs the ground better and typically a tire with a higher sidewall will have deeper tread depth. Most 17's in that size will have 13/32 tread depth where 18" will be more like 11/32's. 18" feel better in dry conditions and are stiffer and more sporty which is what this car needs as it is heavy and somewhat of a lazy, softer character anyhow. IMHO
Back to the OP original question, yes you can run the 18" spare. It will make no difference as long as the ratios are 225/65/17 and 235/55/18 respectively.