I usually get that about one run out of five. I make sure that I do one run, even if is is not a full one, it says in the manual it needs to read some G's to stabilize.
After that find a good level run that is longer than the 1/4 mile. Come to a complete stop and hit the button to start the run and begin the staging. Once the light says your staged punch it and try not to spin too much, the more you hook and exert G force the more accurate the run will be. Once you hit full 1/4 mile run just let off the throttle, do not hit the brakes. You need to coast some for it to stop measuring and record the run properly.
As long as you have the weight in properly you should get a good plot most of the time with that method. I have found that the E.T is close to a few tenths and the trap speed usually is off by 3 to 6 MPH.
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