Eight MPG?!

[Number21]

I've had my '73 for a few weeks now, at first I calculated almost 10mpg, then closer to 9, now I'm at about 8.5MPG. I know these things weren't built for mileage but come on! I used to have a C20 with a 454 that got better mileage than that!

My truck is a K20 with 350/350/NP205, 4.10 gears in the rear and 33" tires. I know the tires aren't helping either, but does 8.5mpg sound ridiculous to anyone else? The only reason I want a small block over a big block is to hopefully get double digits. I drive 95% on the highway at 55mph, which calculates out to about 2300 engine RPM. I checked the odometer with my GPS and it is right on, and this is with a canopy on the back.

I don't know much about the engine other than what I can see from the outside - it's not stock - has an Edelbrock 4bbl carb and intake, headers, and HEI ignition. It starts and idles great and seems to run fine. I plan on replacing the fan (it doesn't even have a clutch) with electric and synthetic oil in the drivetrain...but I don't feel like I should have to have that stuff to get better than 10mpg, am I wrong?! If I can't improve it much I will put in a big block instead. My tailpipes (dual exhaust) are only 1 7/8" ID, would it help if I put larger pipes on?

Also I have an open element air cleaner...I like how it looks, but is that killing the mileage? What does the stock air cleaner look like on a '73, it doesn't have a snorkel like the later models does it? The cap and plugs look ok to me but I'm going to change them anyway...

[donut]

The best I've gotten out of my K-20 was 12, and that was driving like there was an egg under the accelerator. Routinely in the winter it would drop to less than 8 (extended warm-up, plow, winter blend fuel)

Has anything changed recently? Out of tune, bad gas, other items could be contributing (hanging choke?)

imo, the open element air cleaner shouldn't be affecting it unless its clogged or something. (I only changed back to the oem style because of show and ice clogging my air filter)

My truck isn't completely stock. I do still run a q-jet, my headers run into dual 2 1/4" pipes with a crossover, mild cam and Vortec heads. 4.10 gears and 245/75-16 tires (I don't remember if they're 31 or 32").

[72freak]

For a week put some regular size tires on it and dont drive it over 55 and keep your foot out of it and see what you get.

[tucsonjwt]

I have owned 3 squares, all C20, and the worst gas mileage and performance was with the 350/350 combo - in the city was 9 mpg. at best, with that truck. 4:10 gear and 33" tires are like driving a big tractor down the road - so 8 mpg would be good for that vehicle. Most 3/4 ton 4:10 trucks get 9-10 city, in my experience. If you had a diesel you would pick up a few (or more) mpg. Modern diesel 3/4 tons get 17 mpg city, older ones get more mpg but have less power (6.2/6.5).
My current 83 C20 stock 454, granny 4 spd, 4:10, with gear vendor, gets 11 mpg. best in the city when driven very carefully. That truck drops down to 8 mpg city if I get stuck in traffic and do a lot of idling. 13 mpg highway with cruise and gear vendor in use with no A/C on is the best I have ever achieved with that truck. The drivetrain is in excellent condition. Unless you need big tires and engine and 4WD, a 1/2 ton shortbed V6 stick 2WD with open rear end will get you 15-18 mpg city, if the drivetrain is in good conditon - but that truck is more like a car than a truck. I had a 2000 Silverado V6 automatic long bed with 3.73 rear which got 18-19 city, but I drove it very carefully - 15-16 mpg is the EPA rating for a vehicle like that, when driven "normally" like most people would.
Try driving like grandma on her way to church and see if that makes any difference in your gas mileage - likely not though.

[68 TT]

I have the same combo with 5.13 gears with a much more powerful than stock engine and get 7 to 8 mpg in the city.

Highway mileage goes down due to the ridiculous 4100 RPM it sees at 75 mph due to the gearing.

I blame it on the heavy vehicle weight and the small engine having to work pretty hard all the time just to get the heavy pig moving. It just doesn't built enough torque for the application even with a bigger cam, intake, carb, headers etc...

I ran into the same issue with my old 70 Chevelle. While I had the 454 out for a rebuild I dropped in a nice 283 I had sitting around to keep the car on the road. The 283 got far worse gas mileage in the 3900 pound Chevelle than the 454 did and the 454 built substantially more HP & torque, close to triple the torque and double the HP. With the small block you had to get into the gas a lot more just to get the car moving. With the big block it built enough torque just off idle to pull from a dead stop in third gear without lugging down so it took next to no throttle to accelerate normally with it.

I say put the big block in there, get better gas mileage and enjoy the extra torque in the truck more.

[MofoG23]

Thats a bit low...

When I drove my truck home (after I bought it), I drove 200+ highway miles. I averaged about 11-12mpg driving around 65mph.

350 with SM465 and 4:10 gears - completely stock with 235/85/16 load E mud tires...

[PRattenbury]

Put a Cummins 6BT in it, and double your mileage.

[irishman999]

I mention Quadrajet carburetors so much on this forum people are going to start making fun of me. Try putting one on your truck, they are good for mileage.

[clinebarger]

Originally Posted by PRattenbury

Put a Cummins 6BT in it, and double your mileage.

That will do it.

Originally Posted by irishman999

I mention Quadrajet carburetors so much on this forum people are going to start making fun of me. Try putting one on your truck, they are good for mileage.

X2, AFBs run way to rich.

[gchemist]

Buy a vacuum guage. Tune the carb. You live in cold climate. The open element is not that bad. You need warmer air. Too cold of air hurts gas milage. Buy some NGK plugs.

[Number21]

Originally Posted by PRattenbury

Put a Cummins 6BT in it, and double your mileage.

I would kill for one of those...even a 4BT...too bad it cost as much as several thousand gallons of gas...

Well, I guess on the plus side there isn't much I can do to this truck that could make the mileage worse, huh? I have been staying real close to 55mph, and I don't see much traffic since I drive in the middle of the night...

I think I'm going to put the tires from my C30 on it for a few days and see how it goes...I don't need tires that big. The 33s have a pretty agressive tread too, it would probably ride alot better with different tires.

Do you guys think having an overdrive trans would make much of a difference? I've always wanted to put one in one of these trucks...but then again I think of how much gas I could just buy instead. 2300 highway RPM doesn't seem too bad...

Now I'm wondering why GM ever put a six or a 305 in these trucks...if the 350 can barely do it. I do have to give it quite a bit of gas to get going, it doesn't feel like it has a lot of power. Guess I'll have to do something about that.

[Number21]

How about a stock-looking air dam for the front? Think that would help much? (they never had those in '73 did they?)

[87 V20 Scottsdale]

Originally Posted by Number21

I would kill for one of those...even a 4BT...too bad it cost as much as several thousand gallons of gas...

Well, I guess on the plus side there isn't much I can do to this truck that could make the mileage worse, huh? I have been staying real close to 55mph, and I don't see much traffic since I drive in the middle of the night...

I think I'm going to put the tires from my C30 on it for a few days and see how it goes...I don't need tires that big. The 33s have a pretty agressive tread too, it would probably ride alot better with different tires.

Do you guys think having an overdrive trans would make much of a difference? I've always wanted to put one in one of these trucks...but then again I think of how much gas I could just buy instead. 2300 highway RPM doesn't seem too bad...

Now I'm wondering why GM ever put a six or a 305 in these trucks...if the 350 can barely do it. I do have to give it quite a bit of gas to get going, it doesn't feel like it has a lot of power. Guess I'll have to do something about that.

The aggressive tires will hurt you, I went from BFG's to a set of mudders and it cost me 1-2 mpg depending on how I drove. You should have enough engine to get up and going without having to push too far on the gas, maybe something else is going on?

[TheWhiteElephant]

A 350 is not exactly a "small" engine and is certainly powerful enough to power this truck with no problems. I wouldn't go as far as changing the engine to achieve better MPG. Sure, smaller engines work more than larger ones to power the same vehicle, but again, the 350 is ample. 4:10s on 33 inch tires is about right for an efficient performance in the powerband. If you get much smaller tires, you will run higher RPMs. The tread definitely has something to do with it, though, so a good street tire will produce less resistance when compared to an aggressive mud tire.

I wouldn't mess with the more expensive synthetic oil, either. I would perform a complete tuneup including cap, rotor, wires, plugs, etc. and then reassess the situation. Check to make sure the plugs are correct for your engine (hotter vs colder plugs). I believe you should be running R44T or something equivalent. How did you calculate the mileage? As far as I know, these trucks either came with a 16 gallon or a 20 gallon tank, so make sure your calculations are right. An open element air cleaner will typically get you better gas mileage in theory, and the weather in Oregon right now is pretty nice. My '73 K20 came with a snorkel type air cleaner, by the way.

The exhaust seems pretty restrictive, and if the engine is at the very least built a little, you probably need larger pipes. Also, check the CFM on your carb model. Depending on the Edelbrock carb you have, it is likely a Carter AFB copy. These are easy to rebuild so you might even do this if you think it is necessary. You do not need a carb larger than 600 or 650 CFM depending on how modified the engine is. Mild builds are good with this size carb.

The old '77 family Suburban from my youth with a 454/auto got over 9 MPG, so your 8 MPG does sound too low.

M

[79'84'chevy]

i got a 79 1/2 ton with 35's with a 355 small block and a sm465 with 3.73 gears. i get around 12 in town(if i keep my foot out of it, not working now that my distributor works, feels a turbo on it, lol) and like 18 on highway. roughly. speedo is off due to the big tires. work in progress

[gchemist]

http://www.centuryperformance.com/tu...e-spg-148.html

Nothing is simpler to tune an Edelbrock carb!

[Number21]

Originally Posted by 87 V20 Scottsdale

You should have enough engine to get up and going without having to push too far on the gas, maybe something else is going on?

That's what I thought...this truck definately feels slugish compared to my '79 with a completely stock 350, but the 79 also has smaller tires and is 2WD with a manual. Just as an example I'd say I have to give it at least 50% throttle to get going from a stop, also there's a big hill on the highway I drive everyday, if I start at 55mph at the bottom, and don't floor it, I'm down to 50mph at the top. Pedal to the floor I can just barely accelerate up the hill.

Originally Posted by TheWhiteElephant

I wouldn't go as far as changing the engine to achieve better MPG.

Well...that's not the only reason I'd do it...but to me, the only reason to run a small block instead of a big block is to get better mileage. Am I wrong? At least if I had a 454 I'd be having more fun burning all that gas... An 8.1L would sure be nice...I guess riding in style just isn't cheap.

Originally Posted by gchemist

http://www.centuryperformance.com/tu...e-spg-148.html

Nothing is simpler to tune an Edelbrock carb!

Glad to hear that...I need to get a vacuum gauge. I was thinking about maybe taking it to a professional to set the timing and tune the carb, since I don't really know what I'm doing...would that be worth the money? I did manage to time/tune my boat and it has 3 carbs and I had to make the timing pointer...can't be more difficult than that....

[Number21]

Christ, I talked to someone today who drives a full size school bus based on a chevy-something with a 366 big block(?)...he gets 9mpg. Something has got to be wrong with my truck...

[tucsonjwt]

Big difference between a 1/2 ton and a 3/4 ton - like about 1,000 lbs. of extra weight hauling around when empty - has an effect an gas mileage.

[BigBlocksRule]

You have a heavy-ass truck with honkin' wheels and tires, an old engine and all the aerodynamic efficiency of a barn door. You have the recipe for single-digit MPG.
Find a 700R4 core (look for one that has the casting parting line going right through one of the cooler lines, they're the later/better ones). Have it rebuilt locally with a hardened sun shell, 10-vane pump and extra clutches. This should get you somewhere above 10mpg, plus it'll let yous engine work a lot less both getting it going (first is really deep on the 700) and keeping it going with the overdrive. Have the tranny built right and it'll handle the moderate 454 you'll eventually drop in there...

[motornut]

up here the gas milage sucks worse, as the temp drops,
the choke is using more of course,
how bout them secondaries on the carb can you take them off a bit?
i'd get a cover for the front depending how low your temps go,
always seem to get better mileage with the front covered.
better heat, sooner too.
If your winter sees slush and snow and SALT for sure find a stock type cover,
seen the stock units cut up just for winter,
with snorkel/guts removed and matching hole on the other side,

[DetroitIron]

This is a great thread....I've wondered about mpgs before with my 85 K20 (350 with 700R4) and 3.42 gears. I use it just around town and usually avg 11.5 mpg...it is what it is I guess

[Cue-Ball]

Consider yourself lucky. My 1980 half ton shortbox with a 305, TH350, and 2.73 gears only gets a little over 10 mpg. I've got 2WD, a smaller engine, a lighter truck, smaller and narrower tires, higher gears, etc. and I *still* don't get much better than you do!

[gchemist]

Tune the carb. I'm driving a 360+ HP ZZ4 and get 13-15 MPG in the city! 18.5 on the highway!

[Number21]

I'm not sure if I want to say this outloud, but I think I found the problem...

The gas cap is leaking. I finally got my shop cleared out enough so I could park it inside. Geez that thing smells like gas! I thought for sure it was just the exhaust so I turned on my big fan, half hour later it smelled just as bad.

I've been keeping the tank completely full, because my fuel gauge doesn't work, and my bedside is soaked with gas. I guess fuel caps weren't supposed to last 37 years.

Keep in mind, here in Oregon we aren't allowed to pump our own gas ( ) so this isn't something I'm used to checking...