I would like to describe a speedometer. Here are 3 ways I can phrase the sentence:
The dial shows the speed at which the car is moving.
The dial shows the speed which the car is moving at.
The dial shows the speed the car is moving at.
I'm pretty sure the first 2 are equivalent and I'm not sure the 3rd is grammatical, though I do hear it spoken. For the purpose of (current) technical writing, is any of the above preferred?