Hi,
 
As far as I know, the injection locked dividers operate as integer dividers (you simply synchronize the nth harmonic of the oscillator to the input signal). However, to make such fractional divider, ÷2.5, you can multiply by 2, then divide by 5; one way to do that is to make a divide by 5 using flip-flops working on both clock edges. That would be possible at the GHz range if you are using a fast process. The alternative might be using an injection locked multiply-by-2 followed by an injection locked divide-by-5!