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 |
|---|---|---|
| 0 | 1× | |
| 1 | 1.25× | |
| 2 | 1.59× | |
| 3 | 2× | |
| 4 | 2.52× | |
| 5 | 3.17× | |
| 6 | 4× | |
| 7 | 5× | |
| 8 | 6.35× | |
| 9 | 8× | |
| 10 | 10× |
Can't say this is the most useful information, but I found it interesting.