i just bought these RPF1 in 9.5 x 18 with a +38 offset. Is this a correct offset? these will be track only wheels, but they are rubbing a little with the fender on the rear. Tire is 275 35 18..

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Trim the rear fenders and call it good. I put 275/35/19 MPSS on the late model wheels (19x9.5 et 51) and they rubbed on one side. I trimmed the fender to accomodate the increased width.

 

We have members running these. You can probably also play with camber / car height and fix it, potentially. Lance’s suggestion above will guarantee fix it, just paint the metal you trim so it doesn’t rust.

 

I have the same wheels, but I run a 255/40/18 all around, I have not had any rubbing. I went with 255/40's because its the closest to stock tire height. When I was looking into these the most common setup I saw was 265/35/18 and didn't see anyone complain of rubbing

Plugging the specs into a calculator your rear setup is 12 mm (1.8%) shorter and will fill out the wheel well by an extra 6 mm compared to mine.
www.rimsntires.com/specspro.jsp

edit: just curious, what size tires are you running on the front?

 

255 won’t work on the track though. 275 is much more appropriately sized for this weight of car.

 

It would work if you run a slick. Any tread pattern and not so good.

 

Correct offset for your car is 45 front and 55 rear so to answer your question, 38 offset is incorrect.