After you earn a certain amount of total stars, a new truck is unlocked. The total number of stars you currently have isn’t listed anywhere in the game, so you’ll have to keep track of it yourself. The new trucks that are unlocked with Super Excite Mode are a little different: if you have earned past the number of stars required to unlock the Dynamo, then the first SE mode truck (Dynamo) is unlocked after earning 500 more stars, then every subsequent truck is unlocked after the difference between that truck and the next truck is earned. Example: the Mongoose is unlocked after you earn 1000 stars after unlocking the Dynamo, regardless of your over-all star count. The second and final exception for the rule is the Monster Truck and Crazy Monster Truck.

Stars - what unlocks
750 --- Summit
1500 -- Serpent
2500 -- Alpine
3500 -- Raven
5000 -- Dominator
6000 -- Rad
7000 -– Razor

Dynamo:
8000 total stars. If your total exceeds 8000 once Super Excite mode is unlocked, you must earn 500 more stars to unlock it.

1000 after Dynamo ---- Mongoose
1000 after Mongoose -- Blaster
1000 after Blaster --- Mako
1000 after Mako ------ Trekker
1000 after Trekker --- Kodiac
2000 after Kodiac ---- Saguaro
2000 after Saguaro --- Wolverine
3000 after Wolverine - Wasp

When doing an air spin try to keep your wii remote facing towards you so that you can get mroe spins in, and therefore, more stars.

To get alternate paint jobs, you need to race 10 races with the truck you want the paint job for or you can get 3 S Grades with that truck.

When doing an airspin, keep the controller tilted back, many people make the mistake of leaning forward, this just lessens your airtime, causing crashes.

To unlock “Super Challenge mode” in the challenge menu, complete all challenges in “Challenge mode” with S grades.

Once you complete Mirror Mode with all S grades (see hint above to unlock Mirror Mode), every truck that was not previously unlocked will become unlocked.

To unlock the Crazy Monster truck, complete mirror mode with all S ranks.
The Monster truck has every stat maxed out except grip and it's handling rating is slow. The only difference between it and the Monster Truck is it is always invulnerable and never overheats.

To unlock the Monster truck, unlock the alternate paint job for all the other cars. The Monster truck has every stat maxed out except grip and it's handling rating is slow.

Clear all races in excite mode with an S rating

Obtain S-Rank on all tracks on Super Excite race mode

After unlocking Super Excite Mode, get passing grades on all the races in the Bronze, Silver, Gold, and Platinum Cups. Then the Diamond Cup will be unlocked.