78 K20 Gas Milage Suggestions

[learyw]

I'm getting 7 mpg on my stock 78 K20, and I already drive like an old man. I really need to do better than that. Even 10 mpg saves me 30% on gas a year. I can push the truck on flat ground easily in neutral, so I don't think anything is binding up. The truck is in great condition and runs almost perfectly. Here are my thoughts, please weigh in! Maybe I'm missing something else.

1. The rear end is already switched to a 3.73
2. Convert the np203 transfer case to part time with a conversion kit. Awesome transfer case, but I've heard it burns through gas. However, I've heard the conversion (and running it mostly in 2 wheel drive) doesn't even increase mpg that much.
3. Continue to tune my Q-Jet (I just rebuilt it). Maybe buy an air/fuel ratio gauge to get it really dialed in?
4. Switch my Q-Jet to a basic EFI system (I would like to avoid this one)

I love having the 400 transmission, NP203, and original Q-jet. Its about as solid and durable as you can. I'd like to avoid changing it from a stock setup. Thanks!

[geezer#99]

What are you running for timing?

[Ziegelsteinfaust]

My 74 K20 would do 15 highway if I drove 65. City was probably 8-10 but I find it hard to quantify.

I had headers, roller rockers, and a fully tuned Q-jet. Also I bumped my timing to 14* initial. Which worked good with my engine. For rims I used the rotary forged PYO's, and AT tires.

Lastly I had a 14 bolt SF with 3.42 gears. Which is 1/2 the weight of the FF, but also a good 50 or so less rotational weight.

When I first got the truck it was a C20, and had turbine rims. Well they were rotted out, and I swap on 265/75/16's with PYO's. Even with the very heavy 14 bolt FF you could feel the truck had a lot more pick up. I sold the FF, and went to a SF. Even though the FF had 4.56 gears, and the SF had 3.42's. it still felt as lovely as ever. Even with a engine that was dead the mpg went up considerably. When I got it I got about 8mpg. Did a full tune up with bolt one's, and got to 11 mpg. Swapped to lighter rims/tires and it went to 11.5/12. With the 14 bolt SF it would touch 14. The 350 had only about 65-70 cranking psi during this time, but ran like a top otherwise lacking power.

[Matt Man]

x2 on the timing, for the last few weeks I have had my timing at 8 degrees for my smog test and I noticed a pretty big hit in gas mileage, when I usually have it at 14 degrees I have way more power and I usually get around 12-14mpg. I would not change anything to "help" fuel economy usually you will never regain the initial cost of headers or a overdrive transmission. Also make sure your carb is dialed in correctly, choke works properly ect.

[WorkinLonghorn]

Engine size? Freeway/City driving percentages?

Do you find soot in the tailpipe?

Did you check to see if the vacuum advance diaphragm is broken leaving you to drive around with the timing retarded 15 degrees or more?

Your tires are inflated to the proper pressure?


Just the rear end was switched to 3.73??????

Did you put a vacuum gauge on manifold and check this?

Did you check the engine compression with a warm engine, throttle wide open (all plugs removed)?

What about the smog system (if any), is the egr working, is the air pump still installed (if it had one)?

[donut]

Start here, FAQ: SBC Ignition Timing. In case you haven't figured it out yet, a lot (relatively) can be found in the "tune".
Other things that seem minor, but do accumulate. Fresh fluids front to rear, tire pressures (you said stock, so I'm assuming no wide tires), brakes, exhaust...

Like I said, minor, but may squeak out another mile or so.

The '73 I used to own would knock down 13 mpg on a regular basis. Vortec heads, mild RV cam, headers, dual exhaust. Np205, turbo 350, Dana 44, 14 bolt and 4.10's.

[homemade87]

Since you had a gear change are you sure your speedometer is correct . Your calculations may not be what you think they are . If someone else did the work it may not have gotten calibrated correctly . May what to check . Just something to think about .

[learyw]

Thanks for all the responses and questions! I think this is most of them. Im also glad to see people think I can do better than 7 mpg.

-My timing is 9 right now. Ill try bumping it to 12-14? Thanks for the link on sbc. Thats something I can look into.
-The rear end was switched before I bought it, and I only recently noticed it had been changed. I havent checked the front, but I would assume it must also have been switched. I cant imagine it would drive if it wasn't? The speedometer didn't read correctly when I got it because of the swap. I have fixed it and it is calibrated.
-Tires are new and inflated. 265/75R16 with new lighter rims.
-No smog system, I havent really spent time looking at ERG...
-Engine compression is a little low but still good and uniform.
-I do think it might run a little rich. It easily passes emissions each year. Thats why I was thinking about buying a digital fuel/air gauge. Any luck with those?
-Im not sure about the vacuum advance diaphragm. I dont know much about that system. It runs real smooth through all the rpms.
- It does have newer dual exhaust with no cats. Seems to have been done real well.
-Its a 350. I drive 80% city, 20% highway. I am rarely towing.

[geezer#99]

It runs a little rich because of your elevation.
Less air, same fuel.
I'd start initial at 14 and try a couple more or more degrees to find what it likes. Remember you need to start the burn way earlier with less air/rich mixture.
You might find some gains by adjusting your APT.
Info here on that.
http://www.crankshaftcoalition.com/w...4MV_carburetor

[cheepin]

I would convert the 203 to part time and put some lock out hubs up front. I bet you gain a couple mpgs there. Also not a bad idea to install an aftermarket air fuel gauge. This will tell you your mixture while driving. Ideal for mileage would be around 14-1.
Timing I like to run 12degrees at idle.
Since the rear has non stock gearing I would check the speedo to a gps. It's probably off a touch. And city driving will never get very good mileage. It's all stop and go, no good for mileage.

[lilpoindexter]

locking hubs and a 4L60

[70 shorty]

I'd play with the timing as mentioned above, and then I'd get a hand held vacuum gauge and tune for the highest vacuum. Timing is free, assuming you already have a timing light and a vacuum gauge is very inexpensive. I'd do both of those before spending any real money because how long will it take you to recoup whatever larger amounts you spend chasing higher mpg?

[Dead Parrot]

Easy test for vacuum canister. Check timing with canister disconnected(make sure this is how you are setting base timing.) Then connect the canister to a manifold vacuum source(verified with a vacuum gauge). Timing should advance 10-20 degrees(different canisters have different advance amounts). If it does, canister working, if not, time for a new one.

Check the mechanical advance for proper operation. Fairly common for them to quit working after a few decades. If you have the worse case scenario that both the vacuum advance and mechanical advance are not working, will have a big hit on mileage.

Connect a tach to the tach port on your HEI. What are your RPMs at highway speed? Compare to the RPM calculated using your tire size and gear ratio. Too large a difference might indicate a transmission slippage problem.

[Tom]

If you get an a/f ratio gauge it needs to be wide band, otherwise its useless for what you want.
7mpg with a th400, full time 4wd, 3.73's while doing almost all city driving doesn't seem bad.

[learyw]

Thanks, I do have a vacuum gauge and timing light. I dialed it in after I rebuilt the carb. I'm going to advance the timing and re-tune this weekend (as well as try a few other suggestions people gave). Its seems like most people are thinking that is where I am most likely to gain economy. Please feel free to add other suggestions, but Ill let you all know how it goes.

[rich weyand]

I'll second most of the other suggestions, and add a bit:

A wideband A/FR meter works wonders in getting it right. I did all the tuning steps, then bought the meter and found out I was still way off.

Make sure your vacuum advance is on manifold, not ported, vacuum. Ported vacuum will cost you mileage. At idle, there will be vacuum on a manifold vacuum port, but not on a ported vacuum port.

I am running a 78 K10 with 350, TH350, 3.73:1, a modded NP203 with locking hubs, and 31/10.5R15 tires. I get 9 mpg short trips in the city during the winter, and 10 mpg short trips in the city during the summer. The difference is the choke being on and running a richer mixture until the engine warms up. On the highway, I can get 14 mpg at 55, then it starts going down as I go faster. That's all hubs FREE; hubs in LOCK is about 1 mpg less.

[Tom]

Originally Posted by rich weyand

Make sure your vacuum advance is on manifold, not ported, vacuum. Ported vacuum will cost you mileage. At idle, there will be vacuum on a manifold vacuum port, but not on a ported vacuum port.

Ported just means you don't see the vaccum until you crack the throttle open, which is required to move the vehicle down the road, so mileage will not be affected. It will affect your emissions and idle quality though.

[cadillac_al]

Do you think your speedometer/odometer are accurate? Since you had a gear swap something could be off and you are actually getting normal mileage. I have always kept track of my mileage on almost every tank of fuel in my trucks since 2000. My K20 always got 10mpg, never on the highway. My K10 always gets 11 mpg in town or highway. Those are both th350 trucks. With a th400 I would bet they would be 1 mpg less.

Sometimes when I get on a stretch of interstate i like to check my odometer with the mile markers to see how accurate it is. One of my Caddys records 85 miles per hundred miles driven. It makes the mileage seem worse if you don't calculate the odometer error.

[tucsonjwt]

My 83 C20 with 454, granny 4 speed, gear vendor, and 4:10 rear axle gets 9 mpg city and 11 mpg highway. I used to get 11 mpg. city and 13 mpg highway before I replaced the original Qjet with a cheap online replacement.

You cannot expect the same mileage that a 1/2 ton will get because you are carrying and extra 1000 lbs. of vehicle weight.

Everyone I know who has a 3/4 ton old truck is getting about 9-10 mpg in town with a gas engine. Diesel engines get about 17 mpg overall.

To quickly check your odometer accuracy: drive on the expressway and note the odometer reading when you pass a mile post, then note the odometer reading when you pass the next mile post. See how close your odometer reading is to one mile.

Idling in town at stop lights is what kills mpg on the heavy trucks.

[franken]

People used to take out the front driveshaft in the summer on fulltime 4x4 trucks to help mileage.

[cadillac_al]

I just read an article on the new Dodge Ram 3/4 ton truck and that gets 10 mpg too. That's all you can get out of big trucks.

[tucsonjwt]

Here is a link on new 1/2 ton, 3/4 ton, and 1 ton Chevy pickups I posted in another thread - comparing gas mileage among other things.

http://www.herndonchevy.com/Silverado-Comparisons.html

[learyw]

I pushed the timing to 12 deg this morning and re tuned with vacuum. I read a few other posts confirming I need to run around 12 because of my altitude here in Denver. It feels better than it has in the year or so Ive owned it. We'll see if it makes a difference in milage. Either way, I feel like I just got a great kick in power.

[HeavyD]

7!!! Wow you must be a regular at the gas station. Not really an answer to your q but any chance you have room to buy a beater econobox just to run errands?

Imo it will work out cheaper, and then you just enjoy the truck as is (you want the th400 and carb anyway). Just a thought which has probably crossed your mind, but I figured I would toss it out again.

[rich weyand]

Originally Posted by learyw

I pushed the timing to 12 deg this morning and re tuned with vacuum. I read a few other posts confirming I need to run around 12 because of my altitude here in Denver. It feels better than it has in the year or so Ive owned it. We'll see if it makes a difference in milage. Either way, I feel like I just got a great kick in power.

Note that at high altitude, the thinner mixture burns slower and you can go to higher advance. Most people can run an sbc at 16* BTDC at sea level, so you could probably go as high as 18* BTDC.

More is better -- more power, better gas mileage -- as long as you aren't pinging.