Horrible Gas mileage

[Hazieview]

I have a 1987 short wide chevy truck. It now has a small block 400 and a 350 turbo trani. It also has a mild cam and a 650 edelbrock carb and i think a edelbrock performer intake. The truck has a stock rearend (anyone know what it is I forgot.) It also has a stock exhaust (manifolds and 1 muffler with cat)

I am also only using 1 gas tank becuase I had to switch from tbi to carb setup.

I am plan on replacing the two tanks with one big rear suburban tank and dueling the truck out with flowmasters. However I just feel like my gasmilage is horrible. Even worse than when I had the engine/trani in a '66 chevy short/wide bed truck with a .373 rearend. Any ideas on what might help with gas milage or what to check?

Thanks for your time. Kind of hard to have an opinion on this topic but I was just looking for some ideas incase someone has already ran into this problem.

[xtreme80]

Originally Posted by Hazieview

I have a 1987 short wide chevy truck. It now has a small block 400 and a 350 turbo trani. It also has a mild cam and a 650 edelbrock carb and i think a edelbrock performer intake. The truck has a stock rearend (anyone know what it is I forgot.) It also has a stock exhaust (manifolds and 1 muffler with cat)

I am also only using 1 gas tank becuase I had to switch from tbi to carb setup.

I am plan on replacing the two tanks with one big rear suburban tank and dueling the truck out with flowmasters. However I just feel like my gasmilage is horrible. Even worse than when I had the engine/trani in a '66 chevy short/wide bed truck with a .373 rearend. Any ideas on what might help with gas milage or what to check?

Thanks for your time. Kind of hard to have an opinion on this topic but I was just looking for some ideas incase someone has already ran into this problem.

First of all, there were many different gear ratios available for these trucks from the factory. It's impossible to know exactly what you have. If I had to guess, I would say that it's probably got 3.23's or 3.42's in it.

Also, "feeling" like you are getting bad gas mileage, and actually getting bad mileage are two different things. The gauges in these things are just to give a rough idea of where you're at with fuel. Do the math and come up with some real numbers first.

Fill up, and note your mileage on the first fill up. Drive till it shows maybe half a tank, then fill up again. Note how many miles you've driven since the last fill, and divide that by the number of gallons it took to fill it. That will give you the best idea of your true mileage.

[Hazieview]

good point. guess I'll just have to come back with some true numbers.

[chevyrestoguy]

I was always told by fuel experts that the winter blends of gas have a higher percentage of alcohol to eliminate the build-up of condensation in the tank due to the hot/cold (hot engine/cold fuel in the tank) cycles that occur in the wintertime. Alcohol requires more fuel to maintain the same BTU as gasoline, thus lowering fuel mileage. I've never seen statistical data on this, but I have seen decreases of 1-2 mpg on my own vehicles in the winter. Most of my Chevy trucks have gotten around 11 mpg empty or full, so a 1-2 mpg loss is significant.

[piecesparts]

Originally Posted by Hazieview

I have a 1987 short wide chevy truck. It now has a small block 400 and a 350 turbo trani. It also has a mild cam and a 650 edelbrock carb and i think a edelbrock performer intake. The truck has a stock rearend (anyone know what it is I forgot.) It also has a stock exhaust (manifolds and 1 muffler with cat)

I am also only using 1 gas tank becuase I had to switch from tbi to carb setup.

I am plan on replacing the two tanks with one big rear suburban tank and dueling the truck out with flowmasters. However I just feel like my gasmilage is horrible. Even worse than when I had the engine/trani in a '66 chevy short/wide bed truck with a .373 rearend. Any ideas on what might help with gas milage or what to check?

Thanks for your time. Kind of hard to have an opinion on this topic but I was just looking for some ideas incase someone has already ran into this problem.

It appears that you may have some mismatch in your setup. The truck has a 400 with an edelbrock carb and a 350 tranny; do you know what your RPMs are on the highway at 70 MPH? If so, then you can get a general idea of your gear ratio. If you happen to have a gearing of 3.08 or higher, then you could be wasting gas to get up to speed and then maintaining it.

I run a 383 stroker motor, with a 700R4 tranny, Edelbrock AVS 650 carb, Edlebrock intake, and 3.73 gears and the truck gets 15 MPG average. You need to know if your cam and tranny combo are working well together. When you say mild cam, how mild would that be? Does your truck try to push you through the stoplight when you are stopped. (In other words--is the cam radical enough that you had to set the idle higer to make the engine stay running when in gear and the brakes are set) If so, then your torque convertor is at a too low of a stall speed.

What do you have your timing set at? The advance on your timing can affect your gas mileage and motor efficiency. The exhaust can affect your mileage greatly. A lot of engine work with a lousy exhaust will cost you efficiency. Have you done anything to your carb's jets or needles. What model of Edelbrock carb are you running?

[MylilBowTie]

Just a side note if the speedometer doesnt read correct your mileage wont be correct either.

[piecesparts]

Originally Posted by MylilBowTie

Just a side note if the speedometer doesnt read correct your mileage wont be correct either.

That is true, but a person can figure the numbers on how far off it is. With those numbers you can change the percentage of miles driven and calculate the mileage and fuel useage accurately.

[Hazieview]

Originally Posted by MylilBowTie

Just a side note if the speedometer doesnt read correct your mileage wont be correct either.

Yeah that is the main reason I dont have any figures to work with. Didn't want to argue the point though ya know haha. My speedomoter is about 20mph over 'I think' and I dont have a tach to tell my rpms and my timing is good. I do think my truck is idling a little high however my mechanic says it is just fine. It has to be idled up a little bit to stay running but I think it could be cut back a little. I didn't build my engine but my mechanic told me that he put an RV cam in it. He told me the numbers but I forgot, I'll have to ask him. Sounds like I might be able to idle my truck down a little bit and maybe when I duel out my exhaust it might help. I'm trying to buy all new gauges but there so dang expensive I'm trying to hold off. I'm new brilliant big time mechanic as you can tell, I just know basics so Thanks for all the input! Trust me it helps!

[piecesparts]

Originally Posted by Hazieview

Yeah that is the main reason I dont have any figures to work with. Didn't want to argue the point though ya know haha. My speedomoter is about 20mph over 'I think' and I dont have a tach to tell my rpms and my timing is good. I do think my truck is idling a little high however my mechanic says it is just fine. It has to be idled up a little bit to stay running but I think it could be cut back a little. I didn't build my engine but my mechanic told me that he put an RV cam in it. He told me the numbers but I forgot, I'll have to ask him. Sounds like I might be able to idle my truck down a little bit and maybe when I duel out my exhaust it might help. I'm trying to buy all new gauges but there so dang expensive I'm trying to hold off. I'm new brilliant big time mechanic as you can tell, I just know basics so Thanks for all the input! Trust me it helps!

You might try a couple things on your tuning that could help you out. Do you have a vacum gauge or a compound gauge that reads vacuum pretty well? If so, then connect it to your intake manifold and use it to adjust you "IDLE AIR" mixture screws on your carb. The idea is to get the best vacuum setting with the screws and thus giving the best mixture for your idle. After connecting the vacuum gauge, turn the first screw inward until the idle speed drops and the vacuum reading has changed. Now turn the screw outward while watching for the vacuum reading to increase and then steady out at some point. Once found, now turn the screw inward an 1/8 of a turn. Next do the other screw the same way. Once this is done, repeat the steps with both screws and that should have the vacuum adjustment set to the optimum.

Now; did you set your timing with a standard timing light or did you use a timing light with an advance function on it. The advance function allows you to adjust your timng to a "TOTAL" degree setting and that could improve your adjustments and get the most out of your motor. I have my truck set for a total advance setting of 35 degrees, but other motors will work better at numbers in that area. It is some trial and error, but it is worth the work. I used to set my engine tiiing at the settings that the book said, but now I am well above those numbers for an initial timing setting.

As for the idle speed and whether or not it will support the cam. Does the truck have a very distinct lope in it's idle? If the cam is aggressive enough, the idle speed has to be adjusted to keep the motor running and this takes the butterflies in the carb out of the idle air mixture range and into the low throttle postion. If not, then you are probably not impacting the engine with the air fuel mix. With the cam that I am running, I would get a strong raw gas odor at a stop light and the truck was constantly pulling against my brake pedal and then when I could accelerate, it would stumble until I raised the RPMs up to get past the "Stall". I took care of that with a stall convertor and it made a whole new experience out of driving.


20 MPH difference, you need a speedo gear change in the rear of the tranny, pretty bad.

[Thumpin455]

The idle screws wont help mileage much, only if you are idling around with a super rich idle. Sorry but I am no fan of the Edelbrock carb, yes they are easy to install and get running, but fine tuning is a pain and the end result isnt spectacular IMHO. Yes you can get them to run decent and some have even gotten ok mileage out of them. Mopar guys like them because a backfire wont hurt them and Dodge engines are notoriously cold blooded.

I prefer Holleys and Qjets. Holleys work well when the Qjet isnt big enough, like the 700hp 467 Pontiac in my drag car. I like Qjets for my vehicles that get driven regularly because they can be set up to deliver great mileage even on large displacement engines.

The main drawback to a Qjet is the adjustability of the design. If the guy working on it doesnt know how to make them run good, it will be a pain in the rear at the least, and run like crap or not at all, at the worst. If you arent well versed in Qjet building, then you either take the time to learn it, find someone who knows, or you use a different carb and live with the mileage.

Ok that being said, everything has to be working together, the carb, ignition, timing, gear ratio, octane rating, power band of the cam, airflow through the heads and intake, all of it from air ingestion to power at the wheels. The more efficient you make all of it, the better your mileage will be. A more efficient engine will make more power. Brakes that arent dragging or stuck will allow the vehicle to move easier. Freshly repacked bearings will also allow the vehicle to move easier.

There comes a point in the process of making things more efficient that driving style comes into play. It depends on where and how you are driving. In traffic its stop and go, on the highway you want to get in top gear relatively quickly, yet you dont want to bog the engine down by getting there too early.

Open the throttle too far and more air fuel enters the engine, even if it isnt making more noticeable power. That happens alot with 305/350 chevies, they are low on torque compared to some other engines. Partly due to the short stroke and short runner length of the intake. Torque is what gets you moving, so if you build for torque between idle and 4500 rpm you will get better mileage.

So if you have a soggy bottom end and it makes all of its power above 3000, the throttle needs to be opened wide to get going. If you have 300ftlbs or more below 3000 rpm it will move much easier and need less throttle opening to get going. That will save you gas because the engine isnt working as hard.

Take a 455 that makes 500ftlbs of torque for most of its rpm range and a Qjet in a 4100lb GTO with 2.93 gears, an 2004R OD trans, 28" tall tires, that runs mid 13s in the 1/4 mile, and you would think it would get like 10mpg. In fact it got a best of 25mpg following a truck on I40 through Tennessee, and was averaging 21 mpg for the entire trip.

Less wind resistance helped considerably. It was simply traveling farther for each revolution of the engine due to the gearing, and it had the grunt to go up hills or pass, and maintain speed with little or no throttle increase. A more efficient engine makes better power and uses less fuel to do it provided its geared correctly.

You can get a 350 to run good, get decent mileage, and have enough power easily. Its not a slap a bunch of parts together and it works thing. It takes time, effort, a bit of trial and error, and either learning/studying or knowledge. Hell the 540hp engine I had in my drag car got 14mpg highway with 4.10 gears and an 850 mechanical Holley carb. In the 1/4 it would burn through 3 gallons in about 6-8 passes though. It can be done you just need to figure out how to do it, or pay someone who knows. The problem is finding the guys who know.

[streetstar]

Originally Posted by Hazieview

Thanks for your time. Kind of hard to have an opinion on this topic but I was just looking for some ideas incase someone has already ran into this problem.

Buy a Prius and mothball the truck --- 40 mpg

[Ivan D.]

Originally Posted by streetstar

Buy a Prius and mothball the truck --- 40 mpg

Nope, buy a Tahoe or Yukon hybrid 4x4 and run that Prius over! 20mpg city from a 330hp 6.0 V8 engine in a heavy 4x4 vehicle, if that ain't impressive I dunno what is. And by the way, that 20mpg is the advertised fuel economy, which seems to be based on the typical Detroit style of driving (floor the throttle at the green light, speed up to 20 over, floor the brake when tis about to go red), with careful driving much better can be obtained. An added benefit is that you have to be really really talented to get one of these beats to spin out on a winter road, the TCS and VSES systems are wicked good (personal experience). And, best of all, you keep it all GM

[Mickey_D]

Originally Posted by streetstar

Buy a Prius and mothball the truck --- 40 mpg

Or, if you're after mind-numbingly boring extravagant fuel mileage, buy a Diesel Volkswagen Rabbit. 50-60mpg is easily obtainable.

Or for even more interesting....

http://www.mrsharkey.com/pusher.htm