Older guys will understand this pic!

A combination of things could make a Pinto deadly in a significant rear end collision The gas tank could get slammed into the differential and shock mount brackets. These could puncture the tank. The filler neck could be pulled out of the tank, spilling raw fuel all over. The doors could get jammed shut, so if there was an source of ignition for the fuel, the occupants couldn't easily escape. A couple of shields and a longer filler tube greatly reduced the chance of a fire.

What was so damning for Ford was they knew the fix early on. It was also pretty cheap. But Ford did a cost-benefit analysis, and determined it would be cheaper to pay claims for those who were killed than it was to make the Pinto safer. Eventually Ford issued a recall to retrofit existing cars. New ones were built more safely, as they should have been in the first place. Ford sold millions of those cars. They were everywhere.