How to make a linear speed meter?

I am working on an indie game and me and my team are in pre production. I am starting to put together a test build for testing the game mechanics. I have a question regarding GUI: How to make a linear speed meter.

The image below is the gradient showing the speed meter texture.

http://img824.imageshack.us/i/speedgradient.png/

The next image is the backdrop underneath the speed meter.

http://img821.imageshack.us/i/speedgradientbackground.png/

So I want the gradient texture to move according to the player's speed and have the backdrop texture underneath it.

See my excellent question here:

http://answers.unity3d.com/questions/7449/creating-a-circular-progressbar-timer

whose concepts can be directly applied to your problem, though in a non-circular fashion. Simply create an alpha texture gradient similar to the one in that post, though make it in the shape of your "speed meter", and not in a ring.