Against the Speed Freaks one good game plan can be to reduce their mobility as much as possible, including to further emphasise your own. In essence, take down those vehicles so the Orks within are forced to slog it across the field.
Use the heavier weapons to open up the Trukks, then pour Pulse fire into each unit until it is neutralised. Ion Cannons could be good choices here for your Hammerheads if you have the bitz to swop the guns around, also giving you some extra points to use elsewhere.
I'd probably change a few Fusion weapons for Missile Pods if you can - allowing you to stay as far away as possible whilst taking out the vehicles. The Fusion Blasters only really carry the advantage against heavier armour or instant-killing Warbosses/Nobz, and you've still got Railguns lying around there.