If I get a contact moving away from me, I let it go. Not worth the fuel chasing it down, and I know where to find more (unescorted) targets anyway. Unless it's moving "slow" or it's a really juicy target (+10.000 tons), it's not worth it IMHO.
Save the fuel and use it to park yourself at a chokepoint in the middle of a shipping lane. They'll be handed to you on a silver platter.