Honest answers for your Longbed 350 Mileage?

[BIG10]

I sometimes watch the tach and try to keep it under 1500 rpms at shifts points. It's easy in my 5 spd Nissan, but not so easy in the 78.

[Stitch]

buy one with a 6.2 in it and then don't worry about the gas mileage...

[68 TT]

Originally Posted by Benelli

I don't get it....

350 with .06 bore, Weinald Alum Dual plane intake, 650 Holley truck avenger carb, a very, very mild cam, TH350 w/shift kit NP203 T-case with Part time and Hubs, 3.40 gear ratio according to glove box, 31x10.5R15 tires. This is all on a 1979 K10 Longbox.

If I drive like I'm in a damn Buick I get 7 MPG. How in God's name are you guys getting in the double digits? Hell some of you guys get 10+ with a damn 3 speed. I'm jealous. What can I do?

What's wrong? This is mostly 50-55mph 10 miles to town, then 2 miles going 30 and a stoplight here and there, then 1 mile going 30 with slowing down to turn into the school parking lot. I don't romp on it, I usually let it hit 2000 rpms before it changes gears. It's pretty flat out here, I hardly have to "get on it" to get up to speed. It runs like a beast. How much will running rich have on gas mileage? It'll shoot black-fog during cold morning. You can see a spray of black spots in the snow.

HELP PLEASE. I hope I'm not "thread-jacking"

[IMG][/IMG]

HELP.

Looks like you are quite rich on your jetting. Set your air & fuel mixture screws to give you maximum vacuum. That will help a little.

After that see if you can find somebody with a wide band air / fuel meter and get it jetted right.

[westofb]

75 c-20 454 th400 4.11 posi all stock about 4 mpg! I know it is a we bit different than your criteria, but am kind of bummed out about the mpg!

[68 TT]

Originally Posted by westofb

75 c-20 454 th400 4.11 posi all stock about 4 mpg! I know it is a we bit different than your criteria, but am kind of bummed out about the mpg!

Are you sure you don't have a hole in your gas tank?

[westofb]

Originally Posted by 68 TT

Are you sure you don't have a hole in your gas tank?

Nope no hole, engine is just very thirsty! I now have it pulled from the truck to change out the cam, intake, carb, regasket it, and make it pretty. It currently is all original except for the headers and the stock quadrajet and California emissions has a lot to do with the crappy mileage, at least I hope so!

[68 TT]

Originally Posted by westofb

Nope no hole, engine is just very thirsty! I now have it pulled from the truck to change out the cam, intake, carb, regasket it, and make it pretty. It currently is all original except for the headers and the stock quadrajet and California emissions has a lot to do with the crappy mileage, at least I hope so!

Do you have to pass smog where you are at?

It should get much better mileage than that. My 70 Chevelle made 504 hp to the rear wheels with a big block and weighed 3900 pounds yet I was still able to get 10.5 mpg out of it on the freeway. You probably weigh 1000 pounds more but that shouldn't take 6 mpg out of it considering my engine was probably far more thirsty than your truck engine due to the power output.

You definitely have something not working the way it should or a restriction somewhere causing increased fuel consumption. A 454 powered C20 should have no problem hitting 10 mpg.

Headers have a huge impact on big blocks. The stock manifolds on them are very restrictive. You may be running very lean and as a result are getting on the throttle further than you should have to just to get the truck to move. Its possible those huge secondaries on the Q-jet are partially open under normal acceleration in your case causing much of you poor mileage. Get that thing on an engine analyzer when you get the engine back together so you can get the jetting dialed in.

[C-10 simplex]

1) Just for interesting sakes, the silverado extended cab 4X4 i rented 2 yrs ago got 20mpg on the highway.

2) i think the stock q-jet AND stock air cleaner assembly are big keys on paths to getting max mileage---because the air cleaner housing is calibrated to bring in warmer air, which equals leaner mixture etc. i don't feel replacing the manifold, carb and adding an open element air filter are the best moves towards mpg. But more on that later.

More on this interesting topic later.

[68 TT]

Originally Posted by C-10 simplex

1) Just for interesting sakes, the silverado extended cab 4X4 i rented 2 yrs ago got 20mpg on the highway.

2) i think the stock q-jet AND stock air cleaner assembly are big keys on paths to getting max mileage---because the air cleaner housing is calibrated to bring in warmer air, which equals leaner mixture etc. i don't feel replacing the manifold, carb and adding an open element air filter are the best moves towards mpg. But more on that later.

More on this interesting topic later.

Anything you do to an engine to increase air flow through the engine will increase fuel consumption if you tune it to keep the same air / fuel ratio it had before and the intake air temp is the same.

[2bits]

Well, I've had my truck for a few days and put 10 gallons in it and got 120 miles out of it with half lead foot city, and half lead foot freeway driving so guess 12mpg is pretty good. I wonder what my gear ratio is. It is a Big 10 and the glove box sticker doesn't say. I'll have to jack it up some time and spin the wheels.

It's totally true about the over drive tranny being a key, but the truck runs great and if it ain't broke... I have stock tires, on it and I am probably gonna put 275/60s on it too so that will help some.

Thanks for all the input, I am right in line I guess.

[68 TT]

One thing that always improves mileage and gives you more power to the wheels is to put synthetic gear oil in the differential(s).

For the $30 investment per differential it is a noticeable seat of the pants improvement in power and I have seen a 1 to 2 mpg improvement from it too.

While you are in there you can check the ring gear stamping and see what rear gears you really have. Going by the glove box sticker is not always right. My truck says 4.10 by the sticker but I know the original owner put in 5.13's when the truck was only a year old and it still has them.

The next best bang for the buck thing is to replace the motor mounts and transmission mounts with polyurethane. More power gets to the wheels with less wasted moving the motor & trans around. This gets more power to the wheels and you can stay out of the throttle just that much less when driving around with the same end result in acceleration.

A little higher cost but still a nice power and efficiency improvement is to replace the steel driveshaft with an aluminum one. It is worth 5 hp to the wheels on a car with a small diameter short driveshaft so it would be worth a bunch more on a truck with a larger & longer driveshaft. Some of the new body style 88-up trucks, vans & suburbans have large diameter aluminum driveshafts that can be shortened to fit a square body. I found a 1-ton 92 van with a long large diameter aluminum driveshaft that has 1350 U-joints and picked it up for $13. When I do the 4L80E conversion I will have it shortened to fit the 78 K20.

[2bits]

Originally Posted by 68 TT

A little higher cost but still a nice power and efficiency improvement is to replace the steel driveshaft with an aluminum one. It is worth 5 hp to the wheels on a car with a small diameter short driveshaft so it would be worth a bunch more on a truck with a larger & longer driveshaft. Some of the new body style 88-up trucks, vans & suburbans have large diameter aluminum driveshafts that can be shortened to fit a square body. I found a 1-ton 92 van with a long large diameter aluminum driveshaft that has 1350 U-joints and picked it up for $13. When I do the 4L80E conversion I will have it shortened to fit the 78 K20.

I would love to believe it, and I am not arguing as I don't know your knowledge or experience, but you say more than 5MPG can be gained by switching to an aluminum driveshaft? ? ? That's going from 12 17! Is anyone able to chime in on the positive side of that? I'd like to know more!

[463]

Originally Posted by 2bits

I would love to believe it, and I am not arguing as I don't know your knowledge or experience, but you say more than 5MPG can be gained by switching to an aluminum driveshaft? ? ? That's going from 12 17! Is anyone able to chime in on the positive side of that? I'd like to know more!

hold on! it's 5 HP not 5 MPG that he is talking about.

[2bits]

Originally Posted by 463

hold on! it's 5 HP not 5 MPG that he is talking about.

WHEW!! ! Thank you for setting me straight!! I knew I must've been screwed up somehow.

[68 TT]

Originally Posted by 2bits

I would love to believe it, and I am not arguing as I don't know your knowledge or experience, but you say more than 5MPG can be gained by switching to an aluminum driveshaft? ? ? That's going from 12 17! Is anyone able to chime in on the positive side of that? I'd like to know more!

You misread that part. It is 5 horsepower not MPG for an aluminum driveshaft swap. This is a common swap on LT1 and TPI Camaro & Firebird to put in the aluminum LS1 driveshaft in place of the steel part in the older cars. Dyno proven 5 hp to the rear wheels.

I never could stay out of the throttle through a full tank of gas to do a proper mileage check before & after but it definitely went up, just don't know by how much. I would guess about 10 more miles per tank maybe more.

[Shon]

That DS swap did not show up for me at the track....

[68 TT]

Originally Posted by Shon

That DS swap did not show up for me at the track....

I didn't believe it was worth that much either when I heard people talking about it so I did a back to back swap on my 95 Formula that consistently ran mid to low 13.60's when hot lapped at the test & tune sessions.

It ran a 13.645 @ 100.0 at 210 degree water temp with the steel driveshaft and 15 minutes later with the same water temp ran a 13.563 @ 100.6

It had never run in the 13.50's before when hot.

Just under a tenth quicker and over half a mile per hour faster for a grand total of $44 courtesy of the local wrecking yard.

[Shon]

Originally Posted by 68 TT

It ran a 13.645 @ 100.0 at 210 degree water temp with the steel driveshaft and 15 minutes later with the same water temp ran a 13.563 @ 100.6

It had never run in the 13.50's before when hot.

Just under a tenth quicker and over half a mile per hour faster for a grand total of $44 courtesy of the local wrecking yard.

60's on the comparison runs?

[68 TT]

Originally Posted by Shon

60's on the comparison runs?

I don't have that in my excel spreadsheet at work but I don't remember it being much better.

The car did consistent 1.85 60's on the drag radials. If I remember right it was about 1.83 or so on the run with the driveshaft swap.

I put a few more things on the car that day so things changed after that. I got the car into the 13.30's with a handful of new toys (underdrive pulley, K&N cold air intake, subframe connectors, tubular trailing arms, 1.6 rockers & LT4 springs, locks & retainers and a retune to shift at 6300 rpm). The 60's after that were in the 1.70's. It ran a best of 13.04 fully cooled the next weekend.

[Shon]

Cool!
I got my 97 M6 Z28 to 11.80s @ 113 with just bolt ons

Then went to an LS1 and ran 7.33s consistantly with minor bolt ons....

Here is the link
http://www.herronperformance.com/html/shop_car.html

Originally Posted by 68 TT

I don't have that in my excel spreadsheet at work but I don't remember it being much better.

The car did consistent 1.85 60's on the drag radials. If I remember right it was about 1.83 or so on the run with the driveshaft swap.

I put a few more things on the car that day so things changed after that. I got the car into the 13.30's with a handful of new toys (underdrive pulley, K&N cold air intake, subframe connectors, tubular trailing arms, 1.6 rockers & LT4 springs, locks & retainers and a retune to shift at 6300 rpm). The 60's after that were in the 1.70's. It ran a best of 13.04 fully cooled the next weekend.

[68 TT]

Originally Posted by Shon

Cool!
I got my 97 M6 Z28 to 11.80s @ 113 with just bolt ons

Then went to an LS1 and ran 7.33s consistantly with minor bolt ons....

Here is the link
http://www.herronperformance.com/html/shop_car.html

Great toy Shon.

I was using the 95 as my daily driver so I didn't put slicks onto it or a big stall converter that would have really helped the ET for fear of breaking the rear end or trans. I killed the trans anyway after more bolt-on's and getting it to run a 12.78 ET one time. The next run it broke. I never took it to the track again after getting towed home then sold the car after putting in another trans.

My 68 is faster and all the parts are tough enough to not complain at the power level it has so the thing is a fun reliable bracket racer that can hit the streets too.

[Shon]

I have since sold that car and now doing this truck finally.
I am looking at parts to go racing in the truck soon, LOL, I was trying to get out of it but cant.....

[rfmaster]

'75 C20 with clam shell
350 with mild RV cam, headers, 2-1/2 dual exhaust, 300-49 Holley intake, TBI conversion, custom tuning.
Built 700R4 (originally equipped with THM-350)
14 Bolt with 4:11
33" tires

MPG towing a 2500lb car trailer with 3200 lb car in stop and go traffic: 9.5 MPG over 18 miles.
HWY MPG towing an empty 2500lb car trailer at 60 MPH: 14.0 MPG over 45 miles.
HWY MPG empty at 65 MPH: 16.5 MPG.
City MPG has not been verified recently.

//RF

[silverbullet01]

my 1986 chevy, mild 350, 4 barrel, bull low 4 speed, 273 gears, got about 20-22 MPG, depending how much you had your foot in it. now its the same 350 4 barrel, 4 speed, now its 3/4 ton with 4:10 gears, and i get about the same mileage.... sounds like mines a dream on fuel.. haha

[2bits]

Dam. I ask a straight up question about bone stock gas milage, and boom people are talking about freaking drag racing...

Thank you to all the people who replied to the actual question though.

Oh and 68 TT... No offense but I gotta set you straight. I said the glove box sticker did NOT say what gear ratio I had so it would be real hard for me to say it was right when I said it didn't exist, and you also replied to me saying I understood that I misread to tell me that I misread. So read what you are replying to. Thanks