SpeedGauge from Twiscon Software is just that, a speed gauge. Offered over at the Marketplace for $1.99 this app offers a analog or digital speed gauge that feeds off your Windows Phone's GPS.
Speedgauge has a trip counter to measure distance traveled and also tracks your top speed. The settings allow users to switch between kmh, mph and knots (making it handy for boating).
One interesting feature of SpeedGauge is that you can set the display to mirrored which will allow you to reflect the display off your windshield. A make-shift heads-up display so to speak.
In taking SpeedGauge out for a test run, for the most part the app matched my car's instruments. The speed reading matched but there was a one-tenth difference between SpeedGauge and my car (e.g. car read 1.1 miles traveled while SpeedGauge read 1.0 miles). I also noticed at times SpeedGauge had to catch up with my actual speed (likely lag time with the GPS). The analog dial was difficult to read at times (smaller numbers) with the digital display more easy on the eyes.
The app was stable with no hang-ups, glitches or bugs experienced. It does consume power. Each test period lasted approximately fifteen minutes and with nothing else running on my Windows Phone, SpeedGauge consumed 4% of my batter. The app does keep your phone's screen on (you can override this in the settings) and uses your GPS, two major power consumers.
I don't know if I would recommend SpeedGauge solely for automobile use (unless your car lacks a speedometer). I can see it being handy for marine applications where you need to keep track of distances traveled and speed or with other vehicles that do not have on-board instruments (bicycles, ATVs, etc.).
Should you have the need for a speedometer app, SpeedGauge is worth a look at.