There has always been a lot of confusion of what the differences are between Gassers and Altereds, so I thought I would put together a small chart that shows the most important differences between the two.
The fundamental difference is the Gassers (the formal NHRA name is Gas Coupes and Gas Sedans), and Altereds is that the Gas classes were intended for "streetable" cars, while Altered were for non-streetable race cars. But in practice, the two classes had a lot of similarities and a lot of fans and model builders get them confused because of that.
Here is a short listing of the major differences between the two classes. This is not a complete list, and the rules did change somewhat over time. But for the most part, these rules applied through the heyday of the Gassers and Altereds.