Numerous noteworthy Cydia tweaks out there that let customize iPhone’s user interface but few are available for the Maps stock app. Here is a new tweak in Cydia, Speed for Maps, a simple tweak that adds a speedometer overlay, very unobtrusively, in the bottom corner of the stock Maps.app
No new icon is added, but you can configure the output in the settings app. Changing between MPH, KPH, FPS, and even Knots.
Speed for Maps is available free in Cydia. Go get it. This is one of the little things Apple should have thought of.