Ok so I knew the fuel leaking from the car was a new feature added into GTA 5 and was driving around earlier today shooting some cops and 1 of them was leaking fuel which made me get thinking..
Anyway I was curious if there was a certain thing you had to do or place to shoot to create the leak and sure enough i found the spot after only 2 mins of testing.
All you need to do is instead of shooting the engine like most people would assume is the best thing to do you need to shoot just in front of the rear wheels. see the image below for exact spot.
After some more testing it seems you have to shoot that spot but on the left side of the car it may not work if you shoot that spot on the right side of the car
Shooting there causes the fuel leak and if you continue to shoot that area you will be able to instantly set alight the fuel causing the car to explode.
This has worked on 8 different cars ive tryed and worked every time, for best results be sure to shoot from the side exactly ive tryed to shoot while driving and it does work but it takes longer to hit the spot perfectly as you cant always be side on and you up higher while driving.
Go ahead and try it for your self im sure you will be blowing up cars left right and center knowing this. I was going to keep it to myself as it will be very helpful when the online comes out i assume but far to nice to keep it to myself.