i've only ever used the 1/4 & hp calculator in them. Theyre fairly close with ET, but the mph is usually higher as they dont take an average like the track does. That was a few years ago..the newer ones may be better. I wouldnt take what they say as gosple, but for a tuning tool, to base the difference after you add a mod or whatever, then yeah they're handy. I know a few people who take em to the track and say theyre almost bang on.
