Speed for Maps will add a speed meter to the lower right corner of your screen in the native Google Maps application. You can configure it so that it gives you the speed in miles per hour, feet, kilometers, meters per second, knots ... (from the Settings of your iPhone)
The truth is that when I go in the car I use a browser, not google maps, I use google maps on the street to find something and there I don't need to measure the speed, but hey, surely there is someone who seems useful and that is what important.
You can download it free en Cydia.
You need to have done the jailbreak.
iOS 5 beta 5 is here:)… .. waiting for news !!!!!
In which repo?