Preheat the grill to medium-high heat. Brush the corn with olive oil and season with chili powder, cumin, garlic powder, and salt. Grill for 10 minutes, turning occasionally until lightly charred.
In a medium bowl, whisk together the lime juice and olive oil. Add the tomatoes and scallions to the bowl and toss to combine.
Remove the corn from the grill and allow to cool slightly. Cut off the kernels from each cob into a large bowl.
Add the tomato mixture to the corn in the bowl and toss to combine. Serve immediately.