Normal MPG from 400 SBC

[SunTzu]

Hi all,

I have a 71 C-10 SWB Fleetside with a 400 SBC to TH400, 12 bolt 4.11 rear with Detroit Locker. My speedo isn't hooked up but I running at 3000 rpm I am doing about 50-55 mph. I took a road trip through some state hwys keeping the rpms at about 3000 rpm. During the trip I averaged about 7.2 MPG. Is this normal? seems a bit low to me. I had planned on putting in an overdrive transmission and taller gears to get better hwy rpms but is the engine running rich? I know this is hard to diagnose over the internet. Some details. engine was transplanted from a 75 Chevelle or Impala, has Edelbrock 650 cfm carb and Performer Intake, Hedman elite headers, dunno about the internals. I live in Colorado, dunno if the carb is set for High Altitude or not. The tires on now are 255/70R15. The pictures of the engine are just after I got the truck, still has a Nitrous system in place, I have since removed the system. Pictures are all I have of the engine. They are probably not worth anything in this case.

Any input is appreciated, thanks.

Rob

Pics.

[Longhorn Man]

These trucks aren't known for getting great MPG. 10 to 12 seems to be the norm, however, you have low gears and smaller tires (most 4.10 geard trucks are 3/4 ton with taller tires) and that makes it sing on the freeway as you well know.
I would think there's probably a bit you can tune into it... but don't expect much over 10 MPG in that combo.
I just went through your area a month ago, and my MPG was real bad. 402 big block in a loaded down 72 burb that was tuned for Seattle air (sea level) I was blowin black smoke out the tail pipe from it being so rich. I got 9.2 in the mountains... but I did manage 13 in the midwest doin 70 to 75 MPH.

[SunTzu]

Longhorn Man,

What are the specs on the truck you got 13 mpg on. I could deal with that. My DD is a Rubicon with 35's so I am no stranger to the pumps and I don't expect the truck to be a long haul gas miser but 7.2 mpg seems really low.

Thanks

Rob

[DavesRide]

Earlier in my life I drove a 68 with 350/Powerglide. 11 miles to the gallon no matter where I drove it. I hope the current 350/TH400 to get at lest 12mpg. You could get better if you did the 5.3 with all the gizmo's. They do and can run like a striped A$$ ape and have a little econ VS. the Dino carb. I just like mine to consistantly get me where I'm going and don't need the lopety-lope to feel cool about driving a truck.

[uncle]

I had a 70 Impala wagon w/400 sb and a 2 barrel carb (Stock) and got 16/20. Iknow yours is not stock and I had 336 gears.

[jeffspower]

Originally Posted by uncle

I had a 70 Impala wagon w/400 sb and a 2 barrel carb (Stock) and got 16/20. I know yours is not stock and I had 336 gears.

Yep, those were excellent cars. My dad still has my old '71 Caprice 400/2. That thing would get 19-20 hwy. every time running 70-75 mph. I plan on a home for that engine someday.

I had an '80 K20 with a 400/4 & 4.10's. It would get 12 if I drove it nice & kept it under 60.

[Bowtie001]

That seems too low. What would it get if you stayed on a country road at a more leisurely 45 mph? Mine gets 18-19 but with a 350 and 3.08s, but a 4.11 is really not that much of a different. I got my best at around 65 mph... the equivalent RPM for your truck should be 48 mph, so the rpm of the engine is the same at that speed and your aerodynamic losses are actually less. Hypothetically, you should be getting at least 16 at 45 mph, but tuning is a huge deal. If I were you, with that big of a problem (assuming you are not a carb guru) I would but a wideband A/F ratio meter and drive around and watch it. There is a lot you can learn, there was an article about it in car craft or hot rod, and it sounded like the best thing next to a chassis dyno. Without looking at a brake specific fuel consumption curve it would be hard to tell where you should be at 55 mph. Of course the automatic will be losing some efficiency to my manual. Maybe your converter or clutches are slipping?

Another suggestion is to switch to a TH350 with a lockup covnerter - that makes a HUGE difference.

My guess, with headers and a good intake you should be at 20 mpg with a small block. The only exception is that the cam may be weird. You may want to swap to a RV style cam because they are pretty good. Is it real lopey at idle?

One thing I do want to mention: you have a really nice truck. I'll trade mine and then you're gas mileage problem will be solved!

Good Luck
-Brian

[jeffspower]

You can do a "cheapo" A/F tuner trick (like I do ). Weld in an o2 bung. Install cheapo parts store single wire o2 sensor. Attach a wire & run it into the cab. Attach a DVOM to it, and set it to the MV scale. 0 MV is way lean, 1000 MV is way rich. Shoot for 200-400 MV in "leisure" cruising, and 600-800 MV WOT. You will learn quickly if your carb is all- around too rich or if the power valve is coming in too soon. I'd bet your Edelbrock is way rich & all- around out of wack. Buy a jet/ calibration kit & have fun

[Longhorn Man]

I don't think I have ever heard of an old school carb'd small block getting 20 MPG... not in a truck with the aerodynamics of a barn.
My specs are nothing special... 125xxx mile old 402 (after pulling the valve covers and vfinding litterally not one bit of sludge, I wonder it it has been rebuilt at one point), with a later model (mystery) quadrajet with no working choke, TH400, 3.54 gears (this is a 3/4 ton) with a brand new HEI and plugs. I think the MAIN reason i was able to do that, my throttle was messed up. The cable kept falling off the bracket, and that would leave me with somewhere between 1/4 to 1/2 throttle. on flat land, i was only able to get 80 - 82 MPH... did get a gravity assisted 95 MPH... was shootin for the century mark, but had to lift due to a hair pin curve at the bottom of the hill.
In my normal (fixed throttle) mix of city/highway, I've been getting 10 - 12 MPG. I'm not babying it, but I'm not pushing it real hard either.
Personally, I would say the best bang for the buck you can do for MPG, would to be swapping out the rear axle. (easier and cheaper than just a gear swap) I would stick with the tranny you have, and locate a 3.07 gear rear axle. They were standard equipment on any small block/automatic truck 67 - 72. (TH trannys... I can't remember what came with the powerglide trucks) Again, I'm talking C/10 trucks, not 20 series or K series. This will put your 60 MPH RPM at about 2500 - 2600 RPM... Which doesn't sound like much... but it is a HUGE drop in reality.
This would be very close to a 3.73 gear with a TH700R4 tranny as far as cruise RPMs... and that would set you back $2000+ ... even if you were careful.
Some will tell you that a 3.07 is a total dog off the line... but a bone stock 350 will fry the tires for a 1/2 block with these gears. It would be a noticeable decrease in off the line performance, but it'll still tug hard. Really only an issue if you are towing/hauling at or above your truck's limits, or if you race it.
A junk yard rear axle should be no more than 200 bucks, and I would HOPE it would be more like 75 - 125 bucks. Put your rear brakes on the other axle (assuming they are worth keeping) and all is gravy.

[beautimus]

Suntzu,
i am with longhorn man on this one, This day in time 4.11's really don't make much sense on the hiway.(with gas at or near 3 bucks a gallon)
these trucks are nice and have great styling but the aerodynamics are the killer. I can't speak with the athority of some of these guys (fuel curves and such) but i can say this, you seem to have a great setup for performance, it should be fun to drive with a lot of pep to it.
and its also a great setup for poor mileage.
this is my opinion:
4.11's + bad aero + no overdrive = about 10-12 mpg (best case)
3.08's + bad aero +no overdrive = about 14-16 mpg (best case)
3.08's + bad aero + overdrive = about 17-19 mpg (best case)
oh, and for the standard disclamer "ACTUAL MILEAGE MAY VARY"
just my opinion.

[SunTzu]

Everybody,

Thanks so much for the input. Long term I will most likely do the 3.08s and OD. I looked into a local shop for purchase and install of a 700R4. They gave me an over the phone quote of $1200 approx. That usually means closer to $1500 I am sure for unforeseen items. So short term is gonna be the Rearend swap, Kinda hurts to lose the Detroit No-Spin though.

Thanks again all.

BowTie- Thanks for the compliment, as far as the cam. There is no lopeness at all straight thump,thump,thump.

[JAKES 68GMC]

I'm only getting about 9mpg with my 350, not really driving it hard either. at 50-55 she'll sing pretty good to you, any faster and it will just scream. If it had some higher gears, or maybe even some 33" tall tires, I bet my mileage wouldn't be all that bad, cause the motor wouldn't have to turn up as high to go normal speeds. When i was swapping the 350 in i was hoping for 11-13mpg......looks like that ship sailed.

[Jtrux]

With an overdrive, 3.50 to 3.73 gears would be more appropriate in my opinion. My k5 gets like 12 -14, not bad I think with 3.08's and a 4 speed but I drive like an old man. My old ford f100 with a 302, three on the tree and 3.00 gears got 17 city, 22 highway believe it or not.

[NICK3757]

I had an 86 crew cab 1 ton with 4.56s, 35s, and a 6 inch lift and i was getting between 8 and 12 mpg with it. And that was on 93 octane gas.

[weasel29gm]

I agree with Andy, 20 and up I almost would have to buy a tank and let 'em prove it. The best I ever got was 17-18 goin' to Bristol. And in some past MPG threads the ones with overdrive didn't get much better. But I've owned 3 and never got below 13-15 mpg.

[chevy72blu]

7.2 seems a TAD low, but not by much. Well tuned, you will see 8 tops. However, driving the truck at around 2200-2500 RPM will help a lot.

4.11's are a race gear, for the most part.

Doing a gear swap isnt cheap either- the shop I used to work at in Las Vegas (Dans Driveline) charged about 800 or so without an LSD (although they are the most reputable shop in town).


Gorgeous truck and motor, by the way.

[custom72c20]

i get about 9-11 mpg with a 350 and 456 gears in a 72 c20 when my foot isn't through the floor board.

[Longhorn Man]

Originally Posted by chevy72blu

Doing a gear swap isnt cheap either- the shop I used to work at in Las Vegas (Dans Driveline) charged about 800 or so without an LSD (although they are the most reputable shop in town)

That's why I recomend getting a whole rear end. Setting up new gears is often not needed and VERY expensive.

[Cannon007]

I calculated up to about 18mpg avg city, wont take it on the highway cause RPM's would really kill it then. 68 longbed with a 406/TH350 3.73 gears but its got TPI from a vette on it and double hump heads. However it goes lean if it goes over 4k rpm, I dont think the TPI can give it enough fuel. I need to get it tuned though, and an overdrive!

[special-K]

My Highlander gots 10 or a bit better.It`s got a 400,4spd,4.10s,35" tires and I let the torque drive the truck.The only time the revs get up there is hiway driving.But,at 65 it`s just rolling down the road effortlessly,even though the revs are at around 3,500 or so.
This truck gets noticably better milage than my Sierra with 350,TH350,and 33" tires.

[mrein3]

So I have to wonder, if your speedo isn't hooked up, and therefore your odometer isn't working, how do you know you're getting 7.2 mpg?

Here is what I'd do:
Hook up the speedo.

Verify it with some other device (GPS, stop watch and mile markers, etc.)

Correct the speedo if it is wrong.

Once your speedo is correct you need to get some real data:
1. Fill the tank, write down the odometer reading.

2. Drive it until it is down past 1/2 tank or so.

3. Fill it again and write down that mileage and the amount of gas you bought.

4. Subtract the mileage you wrote down in step 1 from the mileage you wrote down in step 3. That is how far you drove (miles). In that many miles you consumed the amount of gas in step 3 (gallons). Divide miles by gallons and you get miles per gallon. There is no other way to accurately figure out your mileage.

7.2 is REALLY bad. I doubt you are getting 7.2 without experiencing other driveability issues.

[impala_631]

ouch

[special-K]

There are milage markers on the hiways and there are signs posted giving ditances to towns.These are accurate enough to figure approximate fuel mileage.There`s always a bit of "fudge" in mileage figures anyway.

[SunTzu]

How I got the approx 7.2 mpg result.

I recorded the amount of gas I used during my trip-approx 22 gallons. When I got home I then "mapquested" the route I took and came up with a round trip of 160 miles. hence the 7.2 mpg. As far as other issues, none. It cranks, starts, accelerates, and holds RPMs just fine. NO surging, no backfires, no smoke. I do need to hold throttle at about half while cranking to get it started, I would assume this is normal though.

I did take off the NOS system though, maybe the PO had the carb set rich for the NOS system? Dunno I never dealt with Nitrous before, but the system had two solenoids, one for the NOS and one for extra fuel, so I am not sure if the carb should still be set rich. I will take a look at the jetting on the carb and see if it is rich or not.

Thanks again,

Rob

[mnunn454]

Originally Posted by jeffspower

You can do a "cheapo" A/F tuner trick (like I do ). Weld in an o2 bung. Install cheapo parts store single wire o2 sensor. Attach a wire & run it into the cab. Attach a DVOM to it, and set it to the MV scale. 0 MV is way lean, 1000 MV is way rich. Shoot for 200-400 MV in "leisure" cruising, and 600-800 MV WOT. You will learn quickly if your carb is all- around too rich or if the power valve is coming in too soon. I'd bet your Edelbrock is way rich & all- around out of wack. Buy a jet/ calibration kit & have fun

I agree. Diagnosing what your average air/fuel ratio is is a great 1st step toward knowing exactly what the cause of the problem is. Also, temporarily add a vacuum gauge with a long piece of tubing. 16-21" of vacuum while cruising would be average depending on the cam you're running.

Pull your plugs. Are they sooty black? If so, that's a sure sign it's rich. The porcelain should be mostly white with a golden brown tip.

I'd guess it's severely retarded timing, stuck, bad or wrong needles/jets in the carb, or a severe vacuum leak. Emergency brakes stuck on?