3:55 with a 26 inch tall tire is what I have ran in past cars and what I put together for my Dart. It's usable on the freeway for reasonable distances. I would not want to do an 8 hour drive with that setup, but a short jaunt on the freeway to the next town for a car show is doable.

Your 275/60-15 tires are taller than what I have used so the 3:55 may be better than the 3:23.