Last spring I ditched the single 3" system on my mild 5.0L 242 and added 2.5" duals all the way back. Seat of the pants confirms an improvement in acceleration, and my around town mileage seems to have picked up by about 1 mpg. I finally got the car out on the road for a longer road trip and confirmed a mileage improvement to go along with the performance.

Did a 650 mile round trip down to Augusta, GA for a car show and to visit my family. Had one tank on the way down of 292.5 miles on 11.15 gallons for 26.2 mpg on the way down. Running 60-70 mph on the cruise control. A net drop of about 800 ft. elevation between Knoxville and Augusta. Averaged 60 mph with two short stops for bathroom breaks.

Thought I might do better than the cruise could. Coming back, I covered the last 233 miles on 8.71 gallons -- 26.75 mpg. That included all of the 'uphill' return trip. I'd let it speed up going down hills, and lose a bit going up -- trying to keep the economy (vacuum) gauge out of the red as much as I could. Most of my previous tanks under similar conditions varied between 24.5 and 25.2 mpg.

The quest is on now for the elusive '27 mpg' tank. A bit of strategic drafting perhaps. God I'm old -- my buddies are all trying to knock 2 tenths off their 60' times -- I'm trying to pick up .2 on my fuel mileage.

Car has a VERY accurate odometer (based on wife's BMW and mile markers). Mild efi 302 (280HP/340 ft-lbs.) with the .625 5th gear and 3.73 gears; 225/50/16 tires. So it's turning over 2200-2500 rpm while cruising.