What is the most affordable one we can purchase? The iPhone apps appear to all be worthless. I installed several and found two didnt even work at all, and the others had a huge margin of error. One didnt even stop when I hit 60mph. It just kept timing all the way through 70+. It's just too difficult to drive well and attempt to accurate measure 0-60 and 1/4 is damn near impossible to figure out manually. Track is not an option.