Carracosta

Speed: 110 (max IVs, positive nature, no EVs)

*Shell Smash: 220*

Speed: 179 (max IVs, positive nature, max EVs)

Shell Smash: 358

Tangrowth

Speed: 149 (max IVs, positive nature, no EVs)

Speed: 218 (max IVs, positive nature, max EVs)

Swoobat

Speed: 290 (max IVs, positive nature, no EVs)

Speed: 359 (max IVs, positive nature, max EVs)

Base stats are deceptive because you need to calculate things like EVs, IVs and natures as well. It's not practical to EV train a Tangrowth in speed since it's so slow, so it's better to invest in defences instead. Carracosta will most likely outspeed Tangrowth.

For Swoobat, speed is commonly EV trained, so it would depend on both Pokemon's IVs as to whether Carrcosta can outspeed it.