My mustang right now gets 19mpg mixed city/freeway driving, mostly city. This is with 1.7 roller rockers, 73mm Mass air meter, cold air induction, stock heads with ported exhaust, 1 5/8" headers, 2 1/2" off road h-pipe and cat-back exhaust, MSD 6A ignition system, super conductor wires, 3.55 gears, T5 5 speed, 70MM throttle body/spacer and Trick Flow Street intake manifold.
Gas mileage after all these modifications was the same as before the mods.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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