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.

[donut]

Couple questions that may help others.
Is the odometer accurate?
What do the plugs look like? clean, fouled, missing on 2...?
Intake sealed? iirc, vortec intakes can be a little finnicky.
Cam work for the rpm range your running?

Got a vacuum gauge? That may tell a little.

[BigBlocksRule]

15mpg is about right, that's what I get in mine in a '97 Tahoe with an LT4 cam. Check timing and plugs, make sure everything's right.

[Jake Wade]

Was your best of 15.2 with the Vortec heads and cam swap?

[meatwagon83]

lots of idling will kill your mileage

[84chevyguyid]

I have an 84 3/4 ton 4x4 equipped with a 400 small block from a late 70's chevy pickup, stock tires, 4 speed stick and 4.11 gears. Driving it 65 it would get around 11 mph. I switched to 3.73 gears and a 700 trans and it moved up to 15 driving it 75. Almost wish I would have left the 4.11's in it as it struggles to roll down the freeway in a headwind, usually just drop it back in third if the wind is blowing. The school district has a 79 c 10 with a 350 and a 350 trans and 4.11 gears and it would get around 12. I live in a small farming community and a trip to town for me is 15 miles to the freeway on a 65 mph road then jump on the freeway @ 75 mph for another 28 miles to the bus dealership for parts. A few years ago the turbo 350 died and I stuck a 700 in it and checked the mileage and it got 18 on that trip. I was amazed at the jump on the 79 and disappointed with the increase on the 84 after the trans and gear change. If you really want to tune the engine, get an air fuel meter and stick on it and see where it reads then tune accordingly with jet/metering rod changes. Just a thought, did your vacuum advance quit? That will drop mileage a bunch if your vacuum advance inst working properly. Good luck!

Mike

[Edahall]

Speed also kills mileage. Fuel economy plunges very quickly once you start pushing over 65 mph on these non aerodynamic vehicles. For myself, I jut leave a little earlier, drive slower and enjoy paying less money at the pump.

[walkerb]

I know its also been a hell of a lot colder down there in TX according to my Dad in Denton. Are you warming/idling the truck up before? That will suck the fuel.

[nekkidhillbilly]

could be the cold in general causing higher idle and all

[cleansquare]

No just start up and go. Trucked seemed perfectly tuned back 8 months ago when I did the head swap. Checked everything and haven't touched it since. Need to check the plugs for color

15.2 mpg was before the vortec head swap. But after the 4L80e.

Like I said, I never hit more than 65 mph.

I'm sure the rain, and nasty headwinds both ways plus all the lights I hit didn't help

[nekkidhillbilly]

Originally Posted by Edahall

Speed also kills mileage. Fuel economy plunges very quickly once you start pushing over 65 mph on these non aerodynamic vehicles. For myself, I jut leave a little earlier, drive slower and enjoy paying less money at the pump.

thats no fun though

[Dead Parrot]

If this was the first time you checked the mileage since the cam swap, I would suspect the cam as the main trouble maker mileage wise. But I would do a couple more mileage checks before doing anything. A slight difference in the angle of the truck can change the amount of gas to reach full by over a gallon. Especially if it was a bit under filled the before the trip and over filled after the trip. This could make a big difference in calculated mileage on such a short trip. On mine, I can sometimes get 21 gallons in to one of my 20 gallon tanks after running it dry if I go to a pump on a bit of a down slope.

[meatwagon83]

that is a good question, how big of a cam did you put in it?

[cleansquare]

Ill get the specs tomorrow on the cam.

I think I'm having issues with the settings on the trans. I am in contact with the tci people who made my ez tcu I'm running.

[gchemist]

You have deep gears. Drop the torque converter lock in to 45+ MPH. Wind with a long bed is a gas hog.

[Edahall]

Try dropping your speed to 55 mph. This is where I get the best fuel economy. With the price of fuel these days, the savings becomes quite substantial.

[BigBlocksRule]

^ that's actually where about 75% of the cars on the road get their best mileage. After that, drag begins to take more of a toll.

[crashz]

I think you need to get a better average. Seems odd that you dropped all of your mileage with the cam /heads swap. I'd keep track of it in similar conditions (weather, route, speed) that you had before when you recorded the 15 mpg. If you are consistantly low, it points to the cam.

Was the carb adjusted to accomodate the new cam? I can see much more fuel being burnt if you are getting more air flow.

Are you using that additional power? Sometimes its hard not to. I find myself speeding around in my 2500HD because it seems to have so much power on hand. Consquently my mileage is only around 12 mpgs.

[cleansquare]

Well if you havent read the thread about "my best day ever" you should. It talks about me fixing my tps sensor last tuesday and then resetting all my values once the tps was right and actually working.

http://67-72chevytrucks.com/vboard/s...d.php?t=619293



Today, got up and drove to work like normal, no crazy acceleration but a steady 70-75 mph all the way to work with converter locked. Way home was lwss steady, lots of idling in parking lot waiting my turn to get the hell out of there, then about 15 uninterrupted hwy miles at 70-75, then hit the daily traffic. Stop, go. Stop, go. For a good 8 miles. Then another 10 milea mixed highway speeds and then in town. Stop go.Filled up.

Calculated miles driven, (verified not only by odometer but also gps) and divided by gallons used, got.......

Drumroll please.......

14.38!

Quite a bit higher than 10.5 when I started this thread! This was with about 85 miles drive. I'm confident I could get better with burning a full tank.

[cleansquare]

Here is a pic of the plug pulled tuesday. Nice and tan, no?

[cleansquare]

Figured out I had been running the wrong plugs. I was running stock 350 plugs, the plugs I needed were for the vortec heads, duh. They put the electrode about half inch further in the cylinder. Huge difference in idle.

Today I filled up. On this tank I drove all day sunday in town. Monday to work and back and today to work and back. Flogged the living **** out of it. Downshifting into second every time I merged into a new highwsy, and 80+ on highways. Drive like I normally drivem

Here's my results. Still had 5 gallons left. Not bad for a heavy c20

[nekkidhillbilly]

thats pretty good

[grs]

Originally Posted by walkerb

I know its also been a hell of a lot colder down there in TX according to my Dad in Denton. Are you warming/idling the truck up before? That will suck the fuel.

Your dad, and I are neighbors. I live in Roanoke, Texas.