Sonic the Hedgehog has been "running around at the speed of sound" for 25 years now. One might think that sometime during that two-decade time period, the answer to just how fast Sonic really is would have definitely been answered by now, but that's not exactly the case.
As with most characters who have changed with the times, Sonic's speed seems to vary wildly from game to game. There's also the issue of comics and various cartoons to consider. Which pieces of Sonic's long history are considered official "canon" and which are not?
Most fans begin by stating that, at the very least, Sonic can run 767 miles per hour, fast enough to break the sound barrier. That's kind of his whole deal, after all.
It's after this point that things get a little trickier. According to the Sonic wiki (which cites the Sonic the Hedgehog comic series) the fastest Sonic ever ran was Mach 15, or 15 times the speed of sound. However, numerous games have contradicted this, at least in words if not in actuality. Sonic Unleashed records the blue blur's max speed at 2,500 mph as he races around town, while the manual for Sonic Adventure DX states Sonic can run 3,480 mph, surpassing Mach 4.
None of that, however, takes into account Sonic's various other forms and power-ups. When the character goes Super Sonic, for example, it's said his speed is greatly increased, but it's never really specified by exactly how much. Then, there are the Light Speed Shoes, which supposedly allow Sonic to run faster than the speed of light. That's pretty darn fast.
So, what would happen if Sonic equipped the Light Speed Shoes while in Super Sonic? Would he travell multiple times the speed of light, across the solar system in an instant? It seems theoretically possible.
The only issue is that none of that ever appears in his games (for obvious gameplay reasons). As broken down by the Game Theorists' YouTube channel in a somewhat scientific manner, Sonic is actually pretty darn slow when it comes to his video game appearances.
The Game Theorists use a cinematic cutscene from the best-left-forgotten 2006 Sonic the Hedgehog game for PlayStation 3 and Xbox 360 to calculate Sonic's height, with the character standing 1.1 meters (around four feet) tall. Using Sonic's height, they then measure the distance of the original Green Hill Zone in the first Sonic the Hedgehog game, estimating the level to be roughly 259 meters. The world record time for completing the level (at least at the time) was 25 seconds. By dividing 25 into 259, the Game Theorists calculate that Sonic's maximum speed in the original Sonic the Hedgehog is a little over 10 meters (33 feet) per second.
That's fast, but it's nowhere near the speed of sound, which is a little over 340 meters (1,115 feet) per second. In fact, Mario, Nintendo's Italian plumber mascot, runs faster in the original Super Mario Bros. than Sonic does in his first game.
What about Sonic Unleashed, the game where Sonic supposedly runs faster than 2,500 mph? The Game Theorists crunched some numbers there, too. By using the amount of time it takes Sonic to run the short end of a city block in New York City at max speed, the channel posits Sonic to be running around 80 meters (262 feet) per second in the 2008 video game. Once again, that's fast, but it's not "running around at the speed of sound" fast.
At the end of the day though, there's only so much science that can be applied to a fictional character, no less a blue bipedal hedgehog that wears sneakers. Sonic's speed in his video games is designed with gameplay in mind, not science. Though comics, instruction manuals and even the games themselves may say Sonic can run upwards of 2,500 mph and into the speed of light, actually depicting that in game comes with obvious complications. Players would be unable to even play a Sonic game if the character moved at the speed of light, and even though Super Sonic supposedly makes Sonic far faster than normal, actually showing that accurately wouldn't make for a fun time (not that Sonic's recent games have been all that fun to begin with).
It's all to say that Sonic is as fast as he needs to be or as fast as fans want him to be. He can run across the face of the Earth in a blink of an eye or break the sound barrier with ease. Or, if you stick strictly to his depiction in his game series, he can run really fast, but nowhere approaching the mythic levels of speed with which he's often attributed. That's part of why Sonic has endured after all these years. Even with hardly a good game in recent memory, Sonic still remains one of gaming's unmistakable icons after 25 years, thanks to his constantly-changing abilities and speed. That's the way it should be, especially if it means we get more (and hopefully, better) Sonic games in the future.