Other commentors have pointed out that Sonic isn't running at his full speed during gameplay, because he HAS to move slow enough for human reflexes to act on obstacles. Perhaps the most objective way to measure his true potential is to have a cutscene where he is running at full speed, though those are pretty rare and in the few I can think of it would be nearly impossible to figure out the physical measurements. However, the measurement of Sonic's height is flawed, since you took a measurement from modern Sonic and applied it to his classic sprite, since Sonic generations revealed that Sonic has in fact grown over the years of his franchise, I even ran the calculations myself which you can review here: http://wakeangel2001.deviantart.com/art/I-m-crazy-and-obsessed-400453569 which made me conclude modern Sonic is actually closer to 4'10." Also Green Hill is a terrible place to gauge Sonic's speed because it is hilly and full of obstacles, you needed to pick a stretch of ground that was flat and Sonic can run at full speed unimpeded. Your measure of his speed in Sonic Unleashed is good, I think the final Shadow boss fight from Sonic Adventure 2 could also work, but we don't have a way to measure how long the road the 2 of them were running on is so go with Unleashed. Actually the best place to accurately measure Sonic's [gameplay] speed is Green Hill Zone...from the DS port of Sonic Generations. There is a spot near the beginning of the level that is at the base of a hill, a nice flat bit of ground that is long enough for Sonic to run for a couple seconds and since it's at the bottom of a hill he is at his full running speed as soon as he reaches it. All you need to do is measure the length of that flat ground and you can easily calculate classic Sonic's speed. Modern Sonic is even easier because there are LOTS of regions of flat ground you can have him run on for a couple seconds at full speed (which is easy to get to because of the boost ability.) Unfortunately I haven't had a chance to measure the distances in the DS Generations levels so I don't really have my own conclusions about Sonic's speed, but all the stuff needed to calculate it is there in that one game!