Update, May 5: Results and winners.

We have a fun contest today for GeekWire followers on Facebook, with a chance to win a prize courtesy T-Mobile USA — a Galaxy S 4G phone.

The background: T-Mobile is holding a Speed Challenge at its Seattle-area stores today through Sunday, daring iPhone users to pit their devices vs. the Galaxy S 4G in a mobile download speed test.

Here’s the contest for GeekWire followers: Guess the average download speeds of the iPhones and Galaxy S 4Gs that will race at T-Mobile’s downtown Seattle store. To enter, put your two guesses (one for iPhone, one for Galaxy S 4G) in the comments on our Facebook page.

T-Mobile will give Galaxy S 4G devices to three GeekWire followers on Facebook — the person who comes closest to the iPhone’s average download speed, and the two people who come closest to the average Galaxy S 4G download speed.

If you haven’t already, be sure to “like” GeekWire to be able to post your entry on Facebook.

Hint: We’re talking Mbps here, and you may be able to inform your estimate by using the SpeedTest.net app on your own iPhone or Android device. We’ll accept entries through the weekend and name the winners next week. Only one entry will be accepted per person, and only entries on Facebook will be accepted. Good luck!

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