In the DC Comics universe, Barry Allen, also known as the Flash, is a superhero who possesses superhuman speed and the ability to move at incredibly high velocities. The exact speed differentials of the Flash throughout his existence in the comics may vary due to different storylines, retcons, and interpretations by various writers. However, I can provide you with a general overview of his speed progression.
When Barry Allen first acquired his powers in the comics, he became the fastest man alive, capable of running at speeds faster than any other human being. In his early appearances, his maximum speed was around the speed of sound, which is approximately 767 miles per hour (1,235 kilometers per hour). Over time, his powers and speed increased significantly.
During the 1980s Crisis on Infinite Earths storyline, Barry Allen sacrificed himself to save the universe. However, he eventually returned in the 2009 Final Crisis storyline. Upon his return, it was revealed that his connection to the Speed Force (the source of his powers) had amplified, granting him even greater speed than before.
In the "Rebirth" era that began in 2016, Barry Allen's speed was further expanded upon. He became capable of entering the "Speed Force Dimension" and accessing an energy source known as the "Still Force." These developments allowed him to reach speeds previously unseen in the comics. At his peak, Barry Allen has been depicted as capable of running faster than light itself, surpassing the speed of time and even traveling through different dimensions.
It's important to note that the Flash's speed is often depicted in relative terms, and the exact numerical values of his speed may not always be consistently provided in the comics. The Flash's abilities are often presented as immeasurable and beyond conventional physics.
Therefore, while it is challenging to provide a specific numerical difference in speed between Barry Allen receiving his powers and his death, it is safe to say that his speed grew exponentially throughout his comic book history, eventually reaching speeds that defy traditional understanding.