It finally ran out of gas going up a hill. Luckily there is a gas station at the bottom so I spun around, coasted back down to the pump and we got some data.
Total distance travelled on one tank was 527km in 13 days of commuter traffic
The tank, empty to full, required 80.1L of gas
The last full tank fill was 58L of gas, so there is 22L of gas that's invisible.
That "invisible" gas in the tank allowed me to travel an additional 127km.
Excluding the 25km or so you normally get once you land on E, 102km is still way too much mileage I'm never seeing without risking it. It's gotta be adjusted.