With military experience he'd know a little about fuel air explosives. Powerful fireworks are sufficient for making such a bomb, though they're technically difficult (you need to trigger the second trigger explosion after the first has spread out the fuel enough to detonate)
I think he was expecting (in Marvin the Martian's words) an earth shattering kaboom but didn't do it right
Done right the amount of fuel would have done serious damage - all the energy released in the fire would have been released instantaneously, with a shock wave
I’m still not sure if it was an intentional car bomb or if he was transporting fireworks to use later that night and something about the car set them off.