This tee has a similar theme to the sweatshirt on Classic 2. This time, Sonic is holding Tails' hand pulling him along as he breaks through checkered 'bricks' wall. The art for this shirt was seldom-used. Look how cute the Tails looks, since this was made in 1993, this must have been before they started corrupting the art of him. The design is pretty dynamic, with flying off brick parts & foreshortening. The logo is large at the top, with the copyright at the bottom right. A nice & collectible old tee.

This shirt is from 1994. It is another of those lesser-seen "2 sided tees" which has a design on the front AND the back. The front has orange/yellow checkers & colored circles/rings. In the largest green one is a finger-waving Sonic portrait. The back has running stock art Sonic (with blur effect added behind him) & the logo. Shirt by Signals Wear.

This is a rare shirt. It's an adult size white shirt from the later 1990s. The front has Knuckles in a circle & says "100% Echidna Wear" while the back has Sonic & says "Official Sega Crew" with the Sega logo. It is unusual because it's printed on both sides, as well as for the fact that old tees didn't usually feature Knuckles solo like that. Where could this be from? Was it an event, or an employee thing? Write in if you know for credit.

 Photo discovered by SonicBoy19

This is a Life-Savers candies promotional Sonic tee. Why is he on there? You can find out about the Sonic/Life Savers promo on the Sonic Foods USA page. This shirt likely went along with that promotion. It's very simplistic looking, but styled a bit after the ring Sonic appears in at the start of the games (only now, replaced with a giant lifesaver, which he's popping out of the middle of) This dates back to the time of their contest, so it is rather old and simplistic. That little mark at the bottom is the copyright. 

 Photo discovered by Lightening

Is this a Sonic 3 tee?
This shirt says "Power Up", in an outline green font, spelled vertically up the left side. It is difficult to see in the photo. The art here is unusual. It's Sonic with nice shading holding up 3 fingers, but the angle is different from all the other stock arts & he is frowning a bit. The background is made up of all the same sprite of him jumping, but on alternating black/white square backgrounds. "Sonic the Hedgehog" is written in green, in a block font at the bottom right. Over all, this shirt is pretty unusual.
The art may be an actual 1-off, or an altered piece, the sprite is uncommon to see, the lack of logo/change of font for the name there is also unusual for the time. This is uncommon. 

This 1990s black adult size tee was made to promote the Sega Channel, however, it uses Sonic to do it, so it appears on Gear. The Sega logo & "Sega Channel" are printed in red, and stacked vertically. Classic stock 'arms crossed' Sonic is printed over most of the logos, but is 'standing' on the Sega Channel one so as not to obscure it.
Look at the fabric tag in the back. It's small and square, likely without a license/generic. This means it's likely the shirt wasn't mass produced for stores, it may have been a promotional, a prize, an in-company item, or something else that wouldn't have reached regular retail shelves. It is likely uncommon.

This classic black shirt has a sort of strange background. It looks like they mashed together 2 screen shots of 2 different zones to make it. The top part looks like it's from the Star Light zone while the bottom has that distinctive lava fountain from the Marble Zone. There's a score / time etc readout on that part as well (under Sonic's front leg) so they were just capturing the whole screen and shoving it onto the shirt to make the design. Look at stock Sonic there though, he's been given a little bit of reflected color (back of the head) maybe to make him look like he fits into the background better? No idea how old this is, but it isn't common if it's taken till 2014 to show up.

This black shirt is uncommon. It has the 'slogan' across the top "Gotta Stride Clyde" (most actual old shirts did not have added words like this) The background is repeated green line art of running Sonic. It has a large graphic of foot tapping stock Sonic, but it's not one of the better ones. Long, super thin shoes, big body & somewhat devolved face (like how the Tails stock art became progressively more mutated over time). This one is a genuine classic item.