Speed is off by 5mph but times are REALLY close. I used it on my mustang and my friends and are within .05 or less of our times at track. I got a new best today of 16.23@88.2 with my Probe today besting my old time of 16.4. Not bad for a 130,000 mile GL with only a cone filter and homemade adapter. I think it is a very useful tool for comparing times after mods and helping you improve your driving skills for better times. Use should easily be able to get down to where I am or better with more practice. I am going to try my Probe out this year at the track to compare when not running my Mustang. Practice makes perfect!!! :)