It's really more up to you. Anything fast fits in a Speed Freeks army. Trukks, Battlewagons, Bikes, Buggies, Deffkoptas, Looted Wagons... if it can carry an Ork, an Ork will ride into battle on it.
The numbers are really up to you, but Battlewagons with Deff Rollas have proven to be quite effective, especially when screened with a cheap unit like War Buggies, or with with a
KFF mech inside to give them a cover save.
Now back to figuring out my own Speed Freeks list!