That's because with the torque converter locked up, the engine uses no fuel. Technically the computer sees 0 l/100km, but there's no good sign for "infinite" US mpg.

As folks around here know, I use metric readings, so that means km, °C, 24 hr clock, l/100km consumption, and I ignore the mph on the speedometer. Legally, I switch over the odometer to miles once a year for the state safety inspection.