I think it is instant, because I have been watching mine do that too. I think that while you are idleing, the truck is not moving so the aMPG is going to drop. Just like when you are moving and take your foot off the gas, your average goes to like 30+. I have seen it drop a lot when I let the truck warm up in the morning and then by the time I get to work it has slowly gone back up.
I hope I am on the right track here! LOL
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07 FX4 SCAB, 4.56 Yukons, K&N 57 CAI, Flowmaster 40 SI/DO, Rhino lined, Westin grille and tail light guards, Kobalt 2500 truck box and the best mod of all.....a Gryphon!
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