I filled up with 91 at the track, 15 miles to empty idled for 20 minutes before I lt ran.wouldn't the turbo make up for some of that altitude by keeping boost up? 2 seconds & 12mph seems to be a drastic drop for altitude. Hopefully OP wasn't using that 85 crap they sell there.
The ECU doesn't care if you run 85 on the way to work making little boost because altitude is apart of the calculator of octane rating. 85 at 5000 feet is roughly 87 at 750ft Above sea level
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