Without Forgeworld (27 Vehicles):
HQs can be whatever you want
3 Mobs of Nobz or
MANz in Trukks or Battlewagons
6 Mobs of Boyz in Trukks (don't forget the Deff Dreads you can take because of the Big Meks)
3 Mobs of any mix of: 3 Buggies, 3 Deffkoptas, or 3 Deffkoptas.
3 Mobs of 3 Killa Kans (cuz walkers are vehicles)
With Forgeworld (41):
2 Big Meks in Junkas
3 Mobs of Nobz or
MANz in Trukks or Battlewagons, more Junkas, OR Mobs of 5 Grot Tanks (THAT'S RIGHT, 5).
6 Mobs of Boyz in Trukks (don't forget the Deff Dreads you can take because of the Big Meks)
3 Mobs of any mix of: 3 Buggies, 3 Deffkoptas, 3 Warkoptas, or 3 Deffkoptas.
3 Mobs of 3 Killa Kans or 3 Big Trakks (in Army Builder as "Big Trakk Squadron")
Pure Dread Mob (23-41):
2 Big Meks in Junkas or 2 Kustom Meka-Dreads
3 more Junkas or Mobs of Burnas in Scrap Trukks
6 Mobs of either Spanna Boyz in Scrap Trukks OR Mobs of 3 Deff Dreads (
DD gives you the most vehicles but cost the most points.)
3 Mobs of 3 Grot Tanks or 5 Killa Kans (once again, go wiff da walkas!)
3 Mobs of any of the heavy support slots. Any of these things will only give you 1 vehicle per slot but I would recommend either the Big Trakks or the Lootas inside of Looted Wagons as dedicated transports. That is unless you want to go pure walker, then you take 3 Mega-Dreads!
Don't forget that Codex: Orks and a Dread Mob can ally with each other as battle brothers for even MORE vehicles in your army. Probably the best way to max your vehicles would be to use Codex: Orks with Forgeworld units. Here is the link to the current version of the Dred Mob rules.
http://www.forgeworld.co.uk/Downloads/Product/PDF/d/Dred_Mob.pdf
EDIT:
PS: If you just max all your vehicles in all your slots you will easily be way into really really high point values (5k+ with Double
FoC and Allies). My post is to let you know of your options.