gas mileage...yeah I know its a truck...

[cleansquare]

Hey guys...truck is a 85 C20, 350, rebuilt by me 4 years ago. I added a vortec heads and a larger than stock cam, (not enough to make the engine lope) and the necessary springs and intake. Has a quadrajet. 1" spacer. Long tube headers, no cats, duals into single in single out super 44 flowmasters.

4L80e trans with converter lockup set at 50mph, factory 4.10 gears. Stock size tires with plenty of tread all aired properly.

Before I installed the 4L80e I calculated mileage at 12.2. 3300 rpms at 70.

After I installed it mileage jumped to 15.2. 2300 rpms at 70.

Its been 8 months since I swapped on the vortec heads and cam and intake...just today I decided to fill up and calculate after a trip to bring my little brother home.

I drove 62.4 miles, took 5.72 gallons to fill back up which comes out to 10.90 mpg.

WTF?

I drove with very slow starts gradually reaching highway speeds, top speed 65 mph, mixture of highway and town...caught a lot of stoplights. On the way south the wind was howling and at 65 degrees it was stuffy in the cab so I had the ac going. Leaving moms 2 hours later the cold front had come thru, 36 degrees outside and headed home in a nasty north wind and very hard rain for most of the trip back.

I normally have the truck around 80-85 mph on my 70 mile a day round trip commute to work and back so I must be getting much worse with that.

Is something out of tune? Plugs and wires are new, dist cap and rotor was done with heads... Did the vortec heads and cam cause it to drop 4mpg? The truck is noticeably louder amd has more compression since the heads but really?

I just thought I was getting way better.