Max possible speed and max sustained speed are two different things.
When most of us talk about max speed, it's the max sustained speed you can do. That's why 55 is considered max top speed for the 6.2/3 speed. You can go faster than that, but you risk destroying the motor if everything isn't 100% (and with the GM blocks the odds of having everything 100% aren't the best.
I'm pretty sure you could probably hit 45 in second gear, but not sure how long you can hold it there without blowing the engine.
Without a tach installed you don't actually know what your RPMs are, and therefore don't know how close you are to maxing them out.