Not so slow leak
 

Prior to leaving on a weekend trip tomorrow (Friday) , I was checking the roadster today and that included the tire pressure. I found the right rear at 20 PSI!

I took it back up to 35 and now, 8 hours later, I've already lost 3 PSI.

I had no prior warning from the TPMS, but I'm not ready to find fault with that yet; I might have picked up something coming home the last time I drove it (Monday evening) and the TPMS didn't have a chance to detect it.

I'm going to take it to a shop tomorrow morning that repaired a tire for me before on another car and hopefully we can get on with our weekend trip without much trouble.

My question is this: If it can't be repaired, with 6900 miles on the OEM tires, would I only need to replace the one, or would I need to replace both rears? I do normal driving.

Personally, I'd just get the one tire if it's needed. Chances are you can just get it patched. The tire store that I go to does what they call a permanent patch and I've never had one fail. They patched my front tire twice almost two years ago and I haven't had any issues.

Did you check the pressure while the tires were cold both times?

You might be able to repair the tire with a patch as well.