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Stock engine MPG?

G

gravedigger472

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I have seen a ton of threads on there about fuel mileage, but none specifically with my trucks specs....

I have a 1981 GMC K1500, stock height, 265/75/16 Wrangler MTR. Stock 305 w4bbl Q-Jet, TH350, 3.08 Gears. I have gutted the cat, (it was clogged and busted) and freed up some airflow in the intake w/custom airbox lid. Driving like a grandpa i still only manage 10mpg.

I was looking on 73-87.com and saw some road test articles from a 1986 issue of Four Wheeler mag. They tested a K1500, shortbed. Had 305, 700r4 and 3.42 gears. 31x10.5 tires. The test truck has a little better final drive ratio than mine, but here is the kicker. The mag advertized an average MPG for the 305 of 17.3 (Highway 20.0MPG, and city 15 in town.)

Does anybody on here have a stock truck AND get numbers close to what those advertised? if not whats your best mileage with a stock truck, and what can i do if anything to improve my mileage?

Motors is all stock. Cast iron manifolds, Q-Jet. Runs pretty good, could perhaps use a tune up (ign timing, coil, plugs, cap/rotor etc.) Am also considering smaller tires (235/75/16 or 245's)
 
When my blazer was bone stock 350/700/3.73/31" i got 14mpg on the freeway and 11mpg around town. Now it is lifted with HOT 350/700/4.56/35" and i get 11mpg freeway and 7.5-8.5mpg around town.
 
My 79, 4" lift with 33 MTR's, GMPP 350HO TH350, 3:07 and electric fans only gets 7-8 town, 10 highway. I am also very disappointed and would like to at least see double digits most of the time. I love driving my blazer but can't all the time due to the MPG and the fact I have to use 91 octane.
 
"little better" final drive is subjective. what drivetrain do you actually have?

if you ask me, sounds like that truck actually had a 6.2L in it :)
 
Final drive ratios of my and test truck
Mine
3.08 X 1:1 =3.08
Theirs
3.42 X .7:1=2.39

My engine is a stock 305. I installed a free flowing air cleaner, and gutted cats, and installed Flowtech Raptor mufflers. Same engine as in the test truck. mine has TH350, theirs 700r4. Same tire size. final drive ratios are only .69 different, as explained above. There is no way that only .69 different high gear ratios will make up for near 10mpg differences (test truck 20mpg highway, mine 10). Thats why im stumped. Cruising at 65 nets about 2200rpm, which i figure is optimal efficiency range. Any other thoughts on why this is? Has anyone gotten at least 16 consistently with a carbed 305?
 
Unfortunately, its not just about RPM. A lot of it has to do with teh amount of load that the motor is under. If you downshift from speed, and spin the motor up to say 4k rpms, it is not using as much fuel as it is if you were climbing a hill at WOT and 4k rpms.

That difference in final drive ratio, as well as tires size, truck weight, and torque curve peaks has a lot to do with your final MPG. Is your speedo accurate? That would throw off your calculation also.
 
yeah my speedo is accurate, i also verify using pre-measured distances when calculating my MPG to ensure accuracy. load at cruise speed should be same as Four Wheelers Test truck.
 
Right off the top the first thing i see is that a TH350 always has slip in the convertor (unless it is a lock-up model) where a 700R4 the convertor will lock up in 4th gear which means more effiecent. Also actual tire diameter plays a big role as well, just because the stated tire size says one thing most tires don't measure the stated size and therefor two different brands of tires are highly unlikely to measure the same.

There are two things working against you already as far as comparison.
 
so what other things can I do that will improve my mileage? swapping to a smaller tire or doing a tune up? any real numbers on headers improving MPG from anyone? im not looking to really gain a ton in the HP dept. just mileage for now.

i mean 10mpg just sucks. I'm used to poor MPG, my 76 C10 has a 472 Cadillac and gets about 10mpg (and requires 92octane), i guess i was just expecting a little stock 305 to do better.
 
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IMHO a 305 is a car engine and not designed to move a 5-6K pound vehicle. Swap in a 350 and it will require less work to do the same job as the 305. On a cheaper option, replace that crappy 3.08 gearing. If you're going to keep the 265/75/16's then i would go with 3.73's but if you plan on going with a larger tire then figure out what tires size before doing a gear swap.
 
Definatly do a total tune up. plugs, wires, cap, rotor ect.
Do a Seafoam treatment to the oil and fuel system. The stuff works good.
Dont think smaller tires will help. 265x16 are 31" tall . Stock k5 came with 31" tires.
Custom air box? Does that mean open elment air cleaner? If so you would be better off with a stock set up with cold air intake. Your engine Breathing hot under hood air will only reduce power and MPG.
You dont really need headers but 2 1/4" dual or free flowing 3"single exhaust & performance muffler(s) will help. Just gutting your cat usually does not do much to improve mileage... sometimes makes things worse.
Tire pressure? Keeping your tires aired up properly will help a lot

Or you can just swap in a 350.
 
yeah i actually am planning on swapping to like a 235/75/16, something smaller than whats on there now. i figure a 28.9" tire will help the effective gear ratio as opposed to the 31.6" currently on there. i really dont drive off road alot, just the occasional mud and snow usage.

perhaps someday a 350SBC and maybe a 700r4 are in the cards, but not for a while. College is expensive man.
 
when i got the truck, the factory cold air tube was not connected, i have been considering custom molding a CAI from fiberglass. The "mod" i did to the air intake was temporary. I simply cut five 2"x4" holes along the front edge of the air cleaner housing. Crude, but it looks cool and is more effective than one small opening. That one puny one from the factory does not look like it will flow any air. And yes, i realize that the hot air is not helping my cause, but a remedy is in the works.

also, has anybody else seen a charcoal filled filter element type thing inside the air filter? i had one, but it was clogged so i removed it, i was just wondering what its purpose was, i have never seen any on the many trucks i have worked on.

it was about 13" diameter, fit just inside the air filter, and was 4"tall, filled with charcoal.

anyways, i removed it, because it was clogged and heavily restricted airflow.
 
gravedigger472 said:
so what other things can I do that will improve my mileage? swapping to a smaller tire or doing a tune up? any real numbers on headers improving MPG from anyone? im not looking to really gain a ton in the HP dept. just mileage for now.
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In a very basic nutshell, unfortunately the means to achieve mileage is less displacement, or diesel. You only need to look at new trucks and how little mileage has changed in the past 40 years to see it. Note that there were cars 30 years ago that could achieve 40-50MPG which even today is "high" mileage, but attained the same way then as now...lightweight, aerodynamic vehicles with small engines. Emissions target is still 14.7:1 for air/fuel ratio, and best power in normal setups is still made around 12-13:1, so nothing has changed there for gas engines. Cut a 350 in half, fuel demand (all else being equal and ignoring things like friction and reciprocating weight) will also be halved.

Less basic, engine design, drivetrain efficiency, fuel delivery, vehicle weight, aerodynamics, and so forth are all factors in economy, of which very few we can meaningfully change in these trucks.

Anything like headers, etc., all increase power by allowing more air into the engine. Unfortunately, this means that to keep the mixture from being lean, you have to add more fuel. More power=more fuel unless you make some very fundamental changes to an engine.

The charcoal piece was for emissions, fuel vapors from the carb were intended to be captured there.

I'm willing to bet these engines can't use more air than the snorkel delivers...engine can't intake any more air than it can exhaust. The CAI on these trucks is likely more than adequate for the exhaust they were equipped with, along with the power they were capable of producing, at the RPM's they were designed to run at. It makes no sense from any standpoint whatsoever to "detune" a vehicle by choking it, especially for the manufacturer who is trying to sell something in a market where power is a selling point.
 
please dont take this the wrong way, as i obviously have no idea how you drive, but i have mileage discussions on the regular on another forum that revovles around my TDI Jetta. The number one way to improve mileage is the right foot mod.

To get my 20+ mpg out of my diesel truck, i air the tires up to 40psi, accellerate slowly, and cruise at 55-60mph. yeah you feel like a speed bump, but it pays. many people complain about poor mileage, and then it comes out that they are driving 70-80 mph. you cant have your cake and eat it too.

In my jetta, to cruise at 70mph instead of 60mph, it costs me 10mpg. havent done enough playing with the truck to see if its a similar relationship.

check out fuelly.com, i use it to track the mileage on all my cars, and it has a rather informative tip section on how to increase mileage.

other than that, i also agree that you most likely need a tuneup, new air filter, new fuel filter, and see where that leaves you.
 
K5dreamer said:
please dont take this the wrong way, as i obviously have no idea how you drive, but i have mileage discussions on the regular on another forum that revovles around my TDI Jetta. The number one way to improve mileage is the right foot mod.

To get my 20+ mpg out of my diesel truck, i air the tires up to 40psi, accellerate slowly, and cruise at 55-60mph. yeah you feel like a speed bump, but it pays. many people complain about poor mileage, and then it comes out that they are driving 70-80 mph. you cant have your cake and eat it too.

In my jetta, to cruise at 70mph instead of 60mph, it costs me 10mpg. havent done enough playing with the truck to see if its a similar relationship.

check out fuelly.com, i use it to track the mileage on all my cars, and it has a rather informative tip section on how to increase mileage.

other than that, i also agree that you most likely need a tuneup, new air filter, new fuel filter, and see where that leaves you.
Click to expand...


The speed thing REALLY comes into play when comparing an over drive trans and a non over drive too (700r4 vs. t350)
 
dhcomp said:
The speed thing REALLY comes into play when comparing an over drive trans and a non over drive too (700r4 vs. t350)
Click to expand...

oh yeah completely forgot about that, especially lockup converter wise.

for example, according to an online rpm calculator, youre looking at a 500rpm difference before factoring in the lockup, wich is worth a few hundred rpm. (i know, i had the 700R4 in my camaro). so call it a 700 rpm drop, which is more than 30% from your truck. at highway speeds, a 30% drop in rpm is more than signifigant.
 
Can't take facts the wrong way. People love to go fast, yet complain about gas prices. Mutually exclusive. :)

Taking the mechanics out of the equation, driving style makes a huge difference on economy. My *car* is rated at 31/33 city/freeway (those are the new EPA calculations that are supposed to better reflect how poorly people drive nowadays), I get 35-40 no matter where it's driven.
 
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