I did some testing and managed to determine that the formula used is
Speed Multiplier = 2^(Simspeed/3)
In other words, every +3 difference in simspeed is a doubling of the simulation tick rate. Here's a handy dandy table for those curious:
|Simspeed||Time Factor*||*For positive sim speeds the time factor is times faster, for negative sim speeds it is times slower|
Can't say this is the most useful information, but I found it interesting.