This week I pulled up at QT and waited behind a mail truck to pump gas. The truck had a CNG "This vehicle runs on clean natural gas" sign on it. He was filling the tank with unleaded. What gives?
Simple....mail carriers don't have to complete their routes if their trucks don't run.
Actually, I think a CNG conversion really makes the vehicle "Flex-Fuel", so it can run anything.
The carrier probably found themselves enough short that they couldn't make it back to the fuel stock provided by the government.
Wild guess.
CNG vehicles run on CNG only. You're not going to get unleaded into a pressurized tank no matter how hard you try.
Could be the vehicle was mis-labeled or had body work done with parts taken from another truck.