Pedal to the Metal Achievement in Skylanders Giants
Pedal to the Metal40
Chapter 14 - Collect all 16 speed boosts in "The Long Hall" section
How to unlock Pedal to the Metal
As you fly about in your stolen autogryo, you'll find magenta energy orbs. These are speed boosts, and each section has a set amount to get. As you collect them, a counter will appear in the top right showing your running total for the area. For this achievement, you need to get all 16 in The Long Hall section.
After completing the objective in the Gear Box area (you need to direct a laser past a giant gear), you'll fly into The Long Hall. Follow the trail of loot, and you should find all sixteen. As soon as you get the last one, the achievement unlocks.
Here is a video by 2-4 Gamerz showing the locations: