Good points, all! I'm amazed at the "rapid response" . . . this forum kicks!
I noted the "Premium Unleaded Only" sticker the first time I fueled up. I asked my dad about it, since he owned the car since it was new. He said he had tried both premium and regular unleaded fuel over the years, and never detected a difference in economy or performance. Anyway, I've gotta fuel up again tomorrow, so I'll (a) use premium unleaded and (b) note how many litres/gallons go in. I'm not sure when I'll have an opportunity to blow a full tank on the highway, but that car'* so fun to drive, I'm sure I'll find an excuse!
So far nobody'* flagged my city-driving distance per tank as totally wonky, so I'll operate on the assumption that things are within normal parameters for now. Still, I'd love to hear what you day-to-day commuters are getting on average per tankful . . .