Have you ever wondered precisely how fast Sonic is in real-life units of spacial and temporal measurement? No? Well, I'm going to tell you anyway. First, let's establish what a meter is. We have a very easy and convenient reference for this. Sonic is canonically exactly 1 meter tall. His standing sprite in all 4 classic games is 39 pixels tall. This means that 39 pixels is 1 meter, but only vertically. Why? Pixel aspect ratio, of course! In NTSC, the Megadrive uses 32:35 ratio pixels. This means that 42.65625 pixels horizontally is 1 meter. Now, what is Sonic's max speed? We know it to be 6 pixels per frame when on flat ground*. So let's take 6 and divide it by 42.65625, we get ~0.14066 meters per frame. Now what is a frame? Not 60hz as you may assume! It's actually ~59.92275hz. So let's multiply those together, we get ~8.42869 meters traversed per second. That result times 60, times 60 again, divided by 1000, and we arrive at ~30.3433 kp/h, or alternatively ~18.85445 mp/h. That's right, Sonic can go full-throttle in a U.S. school zone (20mp/h speed limit) and not get a ticket! So much for the "fastest thing alive", eh? Usain Bolt would quite literally give Sonic a run for his money! *though it is possible to exceed this via downhill movement or speed boosts, but I am going to ignore this since it relies on external factors

Doesn't count because it's a bug, but that's capped at 16 pixels per frame, right? Or at least the camera it would seem. Take the speeds I provided in the OP and multiply by ~2.66667. ... also, is that the Sonic CD jump sound in the clip you provided? What's up with that?

Fun thread. Since Sonic 1 is arguably one of Sonic's slowest games, I decided to go ahead and calculate Sonic's speed in Unleashed, his fastest game. So, how would one go about calculating this? Let's take a look over at the Unleashed preview build. This beta includes a handy distance counter, measured in real-life meters. So from there it becomes simple. (I didn't conduct the actual running in the beta though rest assured, as the speed values are slightly different. I just used the beta to measure distances.) I decided to make Sonic run between these two spots in Windmill Isle, clearly landmarked by when the shadow starts and where the door is. Spoiler: Images Using basic subtraction with the meters in the top right, we can say Sonic will be running 140.6 meters. Now for the test itself, conducted in the final game rather than the proto as stated earlier. Spoiler: Images (sonic crashed into a prop in the second image which is why he's a big explosion aha) Again, using basic subtraction with the timer on the top left, we can identify he ran at top boosting speed for 1.946 seconds. Calculating speed from here is simple, if you passed the 9th grade you should know that speed is equal to the distance ran divided by the time it took you to run that distance. This means that Sonic's top speed in his fastest game is 72.2508 meters/second, or 161.62 miles per hour. This would make his theme song true! He is the fastest thing alive, going more than double the speed of the fastest real-world animal, the cheetah. (Still not actually the speed of sound though smh)

Counterpoint: Metal Sonic, who otherwise appears to be the same height as Sonic, stands at 765.4mm in Sonic the Hedgehog CD, so three quarters of a meter tall. If we factor Sonic Generations in, Metal Sonic is also taller than Classic Sonic. By comparison, Shadow and Silver seem to be the correct height in comparison to Modern Sonic, so Classic Sonic might actually be shorter than that, but let's assume they're the same height for the sake of ease.

I'm not sure I agree with the asterisk. You're already assuming an external factor when you say flat ground - without any external factors (i.e. ground), in S1/2/3K his top speed is infinite (because gravity and no vertical speed cap outside of overflowing the game's capacity to handle the numbers, but we're not counting glitches, right?). However, there's a vertical speed cap in Sonic CD of 16 pixels per frame; combine this with the maximum horizontal movement speed of 6 pixels per frame, and it works out at approximately 26m/s. Even if we assume flat ground, we should include the Spindash and/or Peelout, both of which can accelerate him to 12 pixels per frame (~16.8m/s). With sloped terrain, there's effectively no speed limit (with an infinite downward slope, Sonic can roll down it and build speed infinitely much like the no ground example), so there's no useful information to be found there, but we CAN determine the maximum speed of the camera. In Sonic 1, 2, and CD it can move 16 pixels per frame both horizontally and vertically, working out at approximately 33.3m/s when moving diagonally; in Sonic 3 & Knuckles this is upped to 24 pixels per frame or 50m/s. All of this is assuming the rest of the information in the post is correct.