For those of you who have older vehicles (which should be all of us) and those who are running larger tyres on your rigs I found a free IPhone App called SPEEDOMETER. This app gives you your current speed while driving, altitude, allows you to set speed limits and has mapping and waypoints in it. Now I used this app tonight in my work vehicle which is fitted with a fully certified and calibrated Ballinger Speedometer and this app was exact with the Ballingers speed. So if you want to check to see if your speedo is accurate then I recommend downloading this app.