Awesome App for checking Speedometer Accuracy
 

Awesome HUD for iPhone 3G, iPhone 3GS, iPhone 4, and iPhone 4S on the iTunes App Store

I found this free app for my iPhone. I used it last night driving my '60 F100. You can reverse the app and rest your phone on the dash so the speed reflects into the window.

Discovered my speedometer was roughly 5 mph at 60 mph.

Apps like these are great for our old trucks. Now we just need one to monitor fuel consumption and other things :)