There is a recall on the Pro-M MAF for the 1999 Mustang. Apparently, the first lot of these units were sent to retailers with calibration for a 98 GT. You can return your unit for a free recalibration to Pro-Flow - Contact Pro-Flow @248.399.9223 or on the web:
http://www.pro-flow.com/recall.htm for more info.
The recalibration makes a huge difference!
Good luck!
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99 Laser Red GT
Mods: 70mm TB, Level 10 Chip, FMS 3.73's, Sean Hyland Speedo Recalibration kit, Pro 5.0 Shifter, & Pro-M 77mm MAF.
DynoJet: 242HP/286 Lb/ft
Best 1/4: 13.839@99.1MPH on radials.