For the last two tanks, the first half of the gauge travel was normal, and the last half was precipitous. Here's a chronology.
Trailered the car to Thunderhill.
Gauge was slightly over 1/2 tank.
Ran one ~25min session.
Gauge showed ~1/8 tank.
Over the 6mi drive to town for gas (they were out of 92 at the track) the needle dropped to E and the reserve light came on.
Filled the tank, no topping off, it took about 11.5gal. Cleared the trip meter.
Ran the rest of the day at the track, trailered home.
Got in the car this morning, gauge showed slightly over 1/2 tank, 110 (track!) miles on the trip meter.
By the top of the hill (15mi, 2000' climb), the reserve light was on, and I was near the E mark.
Soft-pedaled down the other side of the hill (8mi, 2000' descent) and filled up, again it took about 11.5gal.
If it does the same thing on the next tank, I'll try the fuse fix, but these aren't the same symptoms, are they?