I get 1-2 mpg worse than my 01 v6 did with gears... so I wouldnt worry to much about it. You should see 19-20mpg~ city/hwy 50/50 mix in the GT which is about what a V6 gets. My v6 managed 22-22mpg and my cobra gets 20/19 and if I only drive in the city... its like 16-17mgp but the gt shouldnt be quite that bad.
__________________ 2003 Cobra Vert (Redfire) #3938 of 5082 @ 05/27/2003
472rwhp/493rwtq -Modification List - Dyno Sheet 2012 Mustang 3.7L M6 (Kona Blue) 2011 Ford Edge Sport (Red Metallic)