Of course, the simplest solution to 2.5 and 3.5 speed is to just alternate speeds each round. First combat round for the 2.5 speed character he acts as 2 speed on 6 and 12. Next round he has 3 speed and acts on 4, 8, and 12. Repeat.
Imagine the fun you have when that villian who acted on 3, 6, 9, and 12 has 5 speed on round 2.