Think about it like this.

The circumference of a tire is it's diameter times pi (3.14159265)

So if your stock tires were 29" tall, their circumference was 91.106". That means that every time the axle rotated, you moved forward 91.106".

33" inch tires have a circumference of 103.672". Now, with 33" tires, you're moving forward 103.672" for every rotation of the axle.

Your speed sensor isn't measuring how far you're actually going forward, it's measuring how many times either the axle or transmission is rotating, and then assuming that each rotation is 91.106", but with the bigger tires, you're really going 103.672"

So you're really going almost 14% faster. So, when your speedo is saying 55, you're really doing about 62.

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2000 Ford Ranger XLT
Extended Cab
3.0 Flex
4x4
Manual Transmission
Offroad Package