gas mileage way down...

[1low4x4]

.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.

 

[chengny]

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

That would be considered really good gas mileage for a 3/4 ton truck with a 350 engine going 70 mph - almost unheard of. You might want to disregard that number.

For more accurate results, try calculating your fuel economy over a longer distance and using as much of a full tank as possible.


11 mpg city/highway - with the A/C on, wet pavement - is normal. You should expect a little better mpg with the vortec heads (10% maybe) but not 15+.

Also, wind resistance is a major factor with the square body design. It is the price we pay for that "look".

 

[HotRodPC]

AC on prolly cost you 1.5 - 2 mpg. And if you're saying the wind was strong, unless it was behind you pushing your way, that's another 3-4 mpg. In these squares, becing square as a brick, they are not aerodymanic at all. Also, I know you're using a speedo adapter for the VSS and such. Are you certain your speedo is still accurate or is it accureate now and maybe it wasn't when you checked the 15.2. If the speedo is turning slower now than before, you're actually getting more miles than you realize.

How about a wheel and tire swap recently? Still running the stock narrow wheels? Keep doing so or your mpg will get even worse. How about new brakes recently? If you've got new tighter breaks that aren't quite broke in and dragging a bit more than usual could be an issue. Most likely a combo of things starting with using the AC.

 

[Jims86]

And just a little extra side note here...OEM lockup in 3rd is usually set at 32 mph, and 4th set at 38MPH......may be worth a try, seeing as your engine has a little more poop to push the truck along. Is that Q jet rebuilt with those plugs sealed up?

 

[1low4x4]

I figured I'd lose acceleration if I locked the converter that low. The truck shifts into 4th at 35mph lol.

The carb was went through when I did the head swap

 

[HotRodPC]

Went thru as in, "Put a Kit in it" or actually remanned it? A reman Qjet needs the well plugs sealed, usuually needs the throttle shafts bushed and the throttle plate trued from warpage or at least checked. If you didn't do those 3 things, you pretty much just cleaned it up and put a band aid on it. Notorious issues on a Qjet.

 

[Jims86]

Went thru as in, "Put a Kit in it" or actually remanned it? A reman Qjet needs the well plugs sealed, usuually needs the throttle shafts bushed and the throttle plate trued from warpage or at least checked. If you didn't do those 3 things, you pretty much just cleaned it up and put a band aid on it. Notorious issues on a Qjet.

Yup...the throttle shafts are the first thing I do, then the well plugs...sometimes they are soo bad, its like dumping buckets of fuel in. Dip everything in a bigass pail of berrymans carb dip for a day, then blow out good with air.

As far as the earlier lockups, just using the OE lockup perameters as guidelines, that engine, combined with the Vortech top end should be ok. Extra fuel is being burned between 4th at 38mph, and the lockup of 50. My burb was very happy(20mpg, with old 86 stock cam, and smogger heads) when I set lockup at 40mph in 4th. I set 3rd gear lockup at 35.

 

[hinkv10]

You really need a much larger spread to base your avg mpg figures. Think 1k miles to be truly accurate. I keep a pen and a little piece of paper in my glove box and record mileage and amount of fuel I put in. Takes 30 seconds and I calculate mileage every time I fill up. When adjusted by 14% to account for the actual mileage driven vs what the odometer says, I get 12 mpg religiously. I have a old 305 with a SMI Quadrajet that runs 100%.

Two things; this winter in the NE has been brutally cold and I routinely drive my truck in single digit weather. The fuel economy has gone down a bit and I've average 10 because of warm ups and the use of 4wd. Stopped snowing for awhile and more highway driving and I'm back up to 12. Second point is that I get gas from a couple different stations/pumps and you don't always get the same amount of fuel in the tank. 6 gallons is not enough to make an average. Half gallon off and you average would be flawed by 10%.

 

[chall]

I've never been a q-jet man, i'd get a holly for that rig!

 

[HotRodPC]

I've never been a q-jet man, i'd get a holly for that rig!

For a daily driver, a WELL SET UP and PROPERLY BUILT Qjet is certianly the way to go. For street/strip combo truck, Holley is probably the way to go. But for this particular truck, nawwww he needs to get the Qjet worked over IMO.

 

[1low4x4]

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 worked. Read here :

http://www.gmsquarebody.com/forum/showthread.php?t=9294

Last night I filled up and drove home. 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!

 

[1low4x4]

I'm confident I could get better with burning a full tank. This was about 85 miles driven

 

[HotRodPC]

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 worked. Read here :

http://www.gmsquarebody.com/forum/showthread.php?t=9294

Last night I filled up and drove home. 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!

That's great. I knew you'd get to 13.5 and hoping to see 14. So you made an improvement on that test of just over 3.75mpg better just by tunning the trans. I knew by the looks of those plugs, leave the motor alone if they all looked like the ones you posted pics of. I'd say you've about got it all dialed in now. Now, what about that gear change???? I'd still say NO. Those 4.10's are helping your around town mpg and you'll suffer if you go to 3.73's. Only way I'd want 3.73's over 4.10s was if I was running an non OD transmission and drove 70mph more often than city driving.

 

[1low4x4]

Just a follow up. In another thread I showrd how I chamged the stock old 350 plugs to rhe newer vprtec plugs for my vortef heads. They were a half inch longer.

Today I filled up, took 15 gallons even. Still have 5 left.

On this tank I drove all day sunday in town, and even pulled the boat about 10 miles. Then mknday and tkday drove to work and back. Drove like a bat out of hell. Downshifting into second gear everytime I merged on another hwy, and speeds of 80+. Drove it like I normally do.

Got 14.14 mpg. Thats pretty damn good for a heavy 3/4 ton going high speeds. Went like 212 miles or so

 

[Stewzer55]

Do you have a smartphone? I use an app called FillUp for measuring MPG, it also caluclates fuel cost over time and long term MPG. It also makes me want to send off the PCM for my Trailblazer because the tune for those adds 10-15% Horsepower and 1-3 MPG (supposedly) as long as you keep your foot out of it. I'm also sick of torque management.