Less than 9MPG - what am I missing?!

[Number21]

I posted about this a while ago but apparently my gas mileage is only getting worse. My gas gauge doesn't work, so I've been keeping track of my mileage and budgeting 10MPG. I just ran out of gas. I'm guessing less than 9mpg right now. (on premium no less!)

My truck is pretty much stock besides the 33" tires. K20/350/TH350 with 4.11 gears. I know it's not built for economy but I really get pissed when I see somebody with a big block crew cab claiming something like 14mpg.

The engine is an old 350 (not stock) with power pack heads, that's why I have to use premium. I have an Eddelbrock Performer intake and carb, and headers, but other than that it seems pretty stock. It seems to run pretty good. I want to change the spark plugs but I took one out and it looks ok. The cap/rotor/wires are new and it has an HEI distributor.

What am I missing? Why am I getting worse gas mileage than a school bus?! I drive like an old lady.

[meatwagon83]

your gears are killing you, if you wanna play, you are gonna pay!

[bk2life]

Why do you use premium? That’s a waste. You have 'power pak' heads? What compression is your motor?
What size is the carb?
Where is your timing set at? When was the last time you had your distributor curve set?
You said your plugs looked alright, what does that mean? Did you take a pic? If no pull one out, take a pic and post it.
You have a 350/350 combo with 4:11s and a 33” tall tires. Not a bad combo, but there is more to MPG to be had. How’s your exhaust? You have headers what’s the rest look like?
You drive like a old lady, swap in a 3:50 rear end you don’t need to be winding that motor up for no reason.
Finally, take your truck somewhere and have it tuned. Reset the timing, check the floats in the carb, maybe re-jet it, do a compression/leak down test (its an old 350 get them rings checked out..)
9 mpgs is sucky. I got 9mpgs in my 1984 454 crew cab dually..

[RandyP]

My 06 Yukon with a 496 (8.1) big block, 4x4 and 4.11 gears gets 9mpg. I would imagine yours should get worse tbh.

[andrewmp6]

Bk2life hit the nail right on the head.

[Zebb]

Once you are certain your motor is doing the best that it can do some old school maintenance to make it roll the easiest that it can. Check the brakes, wheel bearings, tires and tire pressures, and use good lubricants in everything.

[donut]

How wide are the tires?
Ignition timing? (made a big difference on my old K-20.)
Driving style? (65mph on the highway was noticeably better than 70+ mpg wise)

When I got mine it was good for 10. When I sold it 13. It did take a lot of tweaking to pull 13. Though I'm pretty confidant, with my set-up, I wasn't going to get much more without $$$.

[meatwagon83]

i just thought about this, how are your driving habits, are you like my brother and think you have to drive like mario andretti? also how aggressive are you tires? i have found that that does matter(rolling resistance)

[bohmankid]

My 84 gets 7 mpg. It has a zz4 bored to a 355 with 10 to 1 compression and a 750 carb with 35 inch tires. My 86 gmc gets 12 mpg with a 350 and a monstrous camshaft with 410 gears and 35's. My 85 gmc gets 9 mpg with a 350 tbi and 410 gears with 33's. Don't feel so bad gears help out a lot with daily driving so id search into some.
Posted via Mobile Device

[ricks2500gmc]

i always thought i would do better and often wonder how some more mpg's could be had. My combo is a 87 gmc 2500 4x4 with a stock 350 and a 700 r4. 3:42 posi. T b i was sent out and completely gone thru by linder performance, upped fuel pressure to 13 lbs, complete tune up, wires, plugs , cap, rotor and all new sensors, oxygen,tps, idle air, egr, and map. set timing up 2 degrees. tires are 285 75 r 16. its my daily driver and iam not a lead foot... i get 12..
Posted via Mobile Device

[meatwagon83]

Originally Posted by ricks2500gmc

i always thought i would do better and often wonder how some more mpg's could be had. My combo is a 87 gmc 2500 4x4 with a stock 350 and a 700 r4. 3:42 posi. T b i was sent out and completely gone thru by linder performance, upped fuel pressure to 13 lbs, complete tune up, wires, plugs , cap, rotor and all new sensors, oxygen,tps, idle air, egr, and map. set timing up 2 degrees. tires are 285 75 r 16. its my daily driver and iam not a lead foot... i get 12..
Posted via Mobile Device

your pretty close, at first when you stated your combo the first thing i figured was 13-14 so your pretty close

[zook]

410's wow you running light 2 light? why are you running 410 and think you should be getting any kind of mileage? Race truck? how about on the interstate how fast are you driving or should I ask how fast can you go? I would imagine not very fast. take out those 410 gears and send them to me I need them should brighten up my 502. good luck
Posted via Mobile Device

[Firebirdjones]

Lots of things you can do for mileage. It's been gone over before.

First thing I'd do, since you are running a carb, is to get a wideband on that thing and dial it in. Chances are good (most of them I see and tune) that the AFR is richer than it needs to be. You can dial in an AFR idle and light throttle cruise in the 14.7 range (what's needed is different for each vehicle) but something in that ballpark for starters.
Then make sure you are getting a decent amount of ignition lead. I prefer 16-18 initial with another 10-15 degrees of vacuum advance dialed in, and shoot for 32-38 degrees total depending on the engine in question.

With these 2 things done I routinely pick up 2-3 mpg. Every car I've tuned here has needed carb jetting, IFR mods, hanger/rod changes etc...as most people slap a carb on, set the idle mixtures and call it good, but it's not optimized.
A good exhaust system and air intake system will also help.

From there you have to look at other things like cruise rpm, rotational weight, unsprung weight and other areas to pickup MPG and make it easier for the engine.

Heavier wheels and tires kill mpg, as well as wider tires that increase rolling resistance. Swap the mechanical fan for electric is usually worth 1 mpg as some people report. Then you get into more expensive alternatives like overdrive transmissions and gear swaps. Fuel injection swaps will help but also get pricey as now you need software to properly tune it for more mpg. There are tweaks to be made in these computers to pickup more MPG like DFCO tables, lean cruise tricks etc...

Alot of these tricks I use on my own stuff as well, and allows my 79 1 ton with a carbed 502/400 turbo and 3.73 gears to get 11-12 mpg around town and 14 mpg highway. My wifes 2000 SS camaro with a small performance cam and 3.73's gets 21 mpg around town and 30 mpg highway. My 72 blazer is my daily, with a 6.0 LS swap/4L60E and 3.73's on 33" tires gets 17 mpg around town and 22 mpg highway.

[Ski-me]

Originally Posted by RandyP

My 06 Yukon with a 496 (8.1) big block, 4x4 and 4.11 gears gets 9mpg. I would imagine yours should get worse tbh.

I get 10.5 MPG on my 04' Yukon XL, 8.1L and 3:73 gears.....

Blazer gets around 12 or 13.

[INSIDIOUS '86]

An overdrive will help too!

Gas requirements are direct off of compression. Using the wrong gas can hurt.
No point in putting premium in a low compression engine and make it harder for it to ignite
Posted via Mobile Device

[theastronaut]

Originally Posted by Firebirdjones

Lots of things you can do for mileage. It's been gone over before.

First thing I'd do, since you are running a carb, is to get a wideband on that thing and dial it in. Chances are good (most of them I see and tune) that the AFR is richer than it needs to be. You can dial in an AFR idle and light throttle cruise in the 14.7 range (what's needed is different for each vehicle) but something in that ballpark for starters.
Then make sure you are getting a decent amount of ignition lead. I prefer 16-18 initial with another 10-15 degrees of vacuum advance dialed in, and shoot for 32-38 degrees total depending on the engine in question.

With these 2 things done I routinely pick up 2-3 mpg. Every car I've tuned here has needed carb jetting, IFR mods, hanger/rod changes etc...as most people slap a carb on, set the idle mixtures and call it good, but it's not optimized.

Exactly! And you can run even leaner than 14.7:1 (light throttle cruise) if you add in more timing under light loads. Lean mixtures burn slower so you need more advance to start the burn sooner. Up to 16.5 is ok before it starts missing/bucking, and anything leaner than 14.7 will run cooler- the extra unburned air cools things off. I know it's apples and oranges, but I've gotten a max of 36.25 mpg hwy in my '64 VW bug with dual two barrels- up from 24 mpg with the stock single carb. The heads run about 15 degrees cooler.

I'm running an Autometer wideband, but Innovate has a better one for the same price.

[Firebirdjones]

Originally Posted by theastronaut

Exactly! And you can run even leaner than 14.7:1 (light throttle cruise) if you add in more timing under light loads. Lean mixtures burn slower so you need more advance to start the burn sooner. Up to 16.5 is ok before it starts missing/bucking, and anything leaner than 14.7 will run cooler- the extra unburned air cools things off. I know it's apples and oranges, but I've gotten a max of 36.25 mpg hwy in my '64 VW bug with dual two barrels- up from 24 mpg with the stock single carb. The heads run about 15 degrees cooler.

I'm running an Autometer wideband, but Innovate has a better one for the same price.

Yep, I tinker with leaner than 14.7 on a couple of the cars here. Perfectly fine on light loads with no harm to the engine. I didn't get into all that in my post, didn't want people to get the wrong idea thinking leaner was better. I usually base it off the vehicle in question. Heavy trucks meant to do alot of towing, I probably won't get crazy leaning things out. Cars with no rear gear, automatics etc...can be the same way depending on the overall engine combo.
Something else that plays a roll is the type of gas used. Ethanol requires a richer mixture to burn properly. Even 10% added (which most of us deal with) requires an AFR of about 14:1 for stoich. The problem I see is that the 10% advertised varies from station to station so a consistent (SAFE) tune is tricky. I've seen as high as 17% with a simple test kit, and that small change throws the AFR for a loop. For these reasons I tend to stay conservative on customer cars to keep the engine safe under various conditions.
Technically we could go on for days about this stuff depending on how involved someone wants to get.

Funny you mentioned VW's, I'm getting ready to tinker with a sandrail. Single carb 1835. Cooler helps on these, since valve adjustments change with heat and have to be checked frequently on these things.

[theastronaut]

Originally Posted by Firebirdjones

Yep, I tinker with leaner than 14.7 on a couple of the cars here. Perfectly fine on light loads with no harm to the engine. I didn't get into all that in my post, didn't want people to get the wrong idea thinking leaner was better. I usually base it off the vehicle in question. Heavy trucks meant to do alot of towing, I probably won't get crazy leaning things out. Cars with no rear gear, automatics etc...can be the same way depending on the overall engine combo.
Something else that plays a roll is the type of gas used. Ethanol requires a richer mixture to burn properly. Even 10% added (which most of us deal with) requires an AFR of about 14:1 for stoich. The problem I see is that the 10% advertised varies from station to station so a consistent (SAFE) tune is tricky. I've seen as high as 17% with a simple test kit, and that small change throws the AFR for a loop. For these reasons I tend to stay conservative on customer cars to keep the engine safe under various conditions.
Technically we could go on for days about this stuff depending on how involved someone wants to get.

Funny you mentioned VW's, I'm getting ready to tinker with a sandrail. Single carb 1835. Cooler helps on these, since valve adjustments change with heat and have to be checked frequently on these things.

Yeah, leaner isn't always better. Only for light loads/light throttle and when you're able to add enough advance at the same light load conditions to help start the burn sooner. And you can't control what fuel is being used on someone else's car. It's alot easier to do in a lighter car too. Interesting stuff though, and something most guys never do. As you say, most throw the carb on and never tune it.

On the VW, if you use HD aluminum pushrods from Smith Brothers' the pushrods will grow with the engine as it warms just like stock pushrods, keeping a constant valve lash. They run quieter than chromoly pushrods too.

[donut]

Originally Posted by zook

410's wow you running light 2 light? why are you running 410 and think you should be getting any kind of mileage? Race truck? how about on the interstate how fast are you driving or should I ask how fast can you go? I would imagine not very fast. take out those 410 gears and send them to me I need them should brighten up my 502. good luck
Posted via Mobile Device

zook, 4.10's are stock for a '73 K-20. 14 bolt ff rear, Dana 44 front.

[y5mgisi]

You could pick up an easy 2-3 mpg by tuning that edelbrock carb!