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Old 05-26-2019, 05:38 PM   #1
Lancialonnie
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Yet Another Gas Mileage Question

Hey Folks, looking for some input on improving gas mileage. I have read through many posts and am envious seeing lots of people getting mid-teens or better MPG. I have restored my 72 C10 long bed with the boards help over these past few years. It is very comfortable and nice to drive, that is until gas in California reached $4/gallon! The original 307 is getting long in the tooth but it runs well. It was rebuild about 25 years ago but very little mileage has been put on it until the last 4-5 years and it is starting to drink a little oil. I have installed an HEI distro, headers/dual exhaust, FITech system w/rear tank and in tank pump, early style 700r4 tranny and a tonneu cover. Every time I added some upgrade I noticed an improvement in drivability and the gas mileage has improved from where I started but I am still only averaging 9 MPG! Is there anything I am missing or is it time to get a 350 crate motor? I am tempted to do it now but this engine is running really well, adding the FI and an OD tranny just made this truck come to life it is enjoyable to drive now especially compared to how it was stock. Thanks for any input!
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Old 05-26-2019, 06:13 PM   #2
tdangle
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Re: Yet Another Gas Mileage Question

maybe with the upgrades you are driving harder? If you are mainly in the city, those upgrades will only help a little. the OD trans will help on the highway. Also what is your rear gearing? If it is already numerically low (meaning high gearing ratio) you may actually be having to work the motor harder to accelerate and maintain hwy speeds. Years ago I had a Dodge 1/2 ton 318 with 2.94 gearing and a 3 speed auto trans. I changed the rear gears to 3.55 for towing and ended up improving mileage all around.

Good luck and enjoy your truck
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Old 05-26-2019, 07:14 PM   #3
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Re: Yet Another Gas Mileage Question

That does seem low. My Blazer with a tired 350, TH350, 4WD, 3.73 rear and 33 inch tires was averaging a little over 11.

Where do you do most of your driving. In addition to rear gear ratio, what tire size are you running?

Also, is your air/fuel ratio somewhere near 14.7:1? Have you done a compression test on the engine?
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Old 05-26-2019, 07:19 PM   #4
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Re: Yet Another Gas Mileage Question

The truck in my avatar and signature gets 10 MPG in mixed driving. That shell sticks pretty far above the cab. With the old shell it was more like 13 MPG in mixed driving. Your rear gear is potentially the main culprit if it is a 4.11 or 4.56.

And the engine was rebuilt over 100k miles ago, so that isn't helping the mileage. Though it gets off the line nicely, it does consume oil, about 1 qt in 1000 miles.
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Old 05-26-2019, 07:23 PM   #5
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Re: Yet Another Gas Mileage Question

My 2 cents, 72 Blazer 2wd conversion....93 Caprice complete drivetrain 5.7 Tbi..700r4...273 rear gear. 15mpg. Probably would not be good for towing, but for everyday driving it is nice.
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Old 05-26-2019, 09:42 PM   #6
Lancialonnie
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Re: Yet Another Gas Mileage Question

I was never able to confirm the rear end gear but I know it is stock as can be as my father in law bought it new. I believe it should have came with a 3.73 since it was a factory automatic? As far as tires I have 235/55/18s in front 255/55/18s out back. The FITech keeps the mixture a little richer than perfect at around 13.7-14. I commute 6 miles one way with half that at 55+ mph. I haven't checked the compression but will do that to see if I have a bad cylinder. If I could only get it up to 14-15 mpg I could live with that but at 9 mpg it is getting tough to drive that while my other car that gets 25 mpg sits in the driveway! Thanks everyone for your input so far!
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Old 05-26-2019, 10:41 PM   #7
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Re: Yet Another Gas Mileage Question

I'm not really familiar with FiTech, but your A/F ratio should be leaner than that. You say half of your commute is 55 MPH. Is the rest of it stop-and-go?

You need to know your rear ratio. If the SPID is still there, it might have the correct ratio on it. I believe 3.73 was standard with the 307, but it could also be ordered with a 3.07 or 4.11 ratio. It wouldn't be a bad idea to open it up and see, since that gear oil probably hasn't been changed in many, many years. You'll find two numbers stamped into the ring gear; divide the larger number by the smaller number and that's your ratio.

Here's another thought - your current tire size is 2" larger in diameter than the stock tire. Have you calibrated the speedometer/odometer for your current tire size? That would definitely affect your fuel mileage calculations.

On the other hand, if you only commute 6 miles, it would take a whole lot of trips to pay for something like changing your rear end ratio.
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Old 05-27-2019, 08:04 AM   #8
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Re: Yet Another Gas Mileage Question

Yeah, you definitely should be getting better than 9 mpg. Time to start checking things out. I agree - start with rearend gear info. If u don't want to r n r cover and change 90 wt. Gear oil ( which i would) just do the tire rotation method. It takes 2 people. Jack it up, mark driveshaft location against rear housing, ( chaulk works good ) . While helper slowly rotates wheel/tire 2 TIMES, count # of driveshaft rotations. It probably will be with a 1/3, 1/2, Etc. Addition to a whole #. Post that # and we can tell u what rear gear u have. Hope i got that right and hope it helps ! Also notice direction of both wheels. If both move in same direction, u have a posi-traction rearend. If one wheel rotates in one direction and the other one goes in the opposite direction, then it's not a posi.
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Old 05-27-2019, 08:28 AM   #9
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Re: Yet Another Gas Mileage Question

I would think you would be getting higher MPGs , I have a 68 c10 daily driver ,when I first bought it it had a 1981 4 bolt 350 .endelbrock carb,373 rear with a TH400 ,I ran it everyday 140 plus miles round trip to work ,with a average of 16 MPGs ,I put about 50,000 on that set up before the head gasket blew, rather then fixing a unknown total milege engine ,I replaced it with a L31 long block crate engine from GM, I also installed a fitech FI system , put a rebuilt TH 200 4R trans in her, and then rebuilt a 71 3.07 rear and stuck that in. my average MPGs are 18 ,but I do drive it at a much higher speed on my 140 plus mile round trip to work and we are talking all highway miles here the truck hardly sees under 50 MPH and 1/2 of that trip im in the 75-80 MPH range . my experience has lead me to think if you are looking for better MPGs when it is time to replace the engine opt to go with a GM LS series engine the LS conversion seem to offer a all around upgrade to these trucks,it does cost more and a little more labor intense but it seems its the way to go these days.
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Old 05-27-2019, 11:53 AM   #10
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Re: Yet Another Gas Mileage Question

I think most automatic trucks had 308 gears . Straight drives had the 373.If he bought the truck new the tag should be on the rear if no one has had the cover off.
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Old 05-27-2019, 02:39 PM   #11
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Re: Yet Another Gas Mileage Question

Have you had the distributor dialed in on a distributor machine? Is your vacuum advance working correctly? Unless you have a real loss of compression issue I would look closely at the timing. Start with the ignition and get that dialed in and if no improvement is seen move to the camshaft timing. A retarded cam will never allow the engine to run efficiently.
Our commutes are very similar. Last year I went from a tired 305 with 3.07 gears in my Burban to a new 350 with 3.70 gears. The same T400 and carb before and after. I saw no change in fuel mileage. 12 mpg for the back and forth to work and 14.0 to 14.9 for straight highway. We did recurve the distributor for the new gears.
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Old 05-28-2019, 04:27 PM   #12
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Re: Yet Another Gas Mileage Question

My '82 K20 with 3.42 gears always got 14ish MPG around town since I got it at 42K miles. The cam went down at 150K and I swapped the tired 350 for a TPI 350 out of a '91 Corvette. I get 17 or so now around town and 20+ on the road. It had a 700R4 from new.
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Old 05-28-2019, 04:57 PM   #13
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Re: Yet Another Gas Mileage Question

Your driving a classic truck , Who cares what the gas mileage is ?
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Old 05-28-2019, 06:11 PM   #14
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Re: Yet Another Gas Mileage Question

Iím with Grumpy!
At her very best, mine gets about 13 or maybe 14 on the highway. Around town, I can just about watch the gauge move.
If I wanted mileage, Iíd get a new truck, with an anemic six cylinder, and look like every other schmoe out on the road.
You have a classic naturally aspirated engine, and old technology drivetrain, so it ainít gonna be efficient.
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Old 05-28-2019, 07:09 PM   #15
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Re: Yet Another Gas Mileage Question

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Your driving a classic truck , Who cares what the gas mileage is ?
That's what Hondas are for.

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Old 05-28-2019, 08:49 PM   #16
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Re: Yet Another Gas Mileage Question

FWIW, my wife's econobox was in the shop awaiting a wiring harness or two, and she was driving my econobox to work. I rented a new Suburban (I neglected to check around for an econobox and just walked in to a rental place) and the thing got 7 MPG.
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Old 05-29-2019, 06:59 AM   #17
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Re: Yet Another Gas Mileage Question

My 350, 3 speed is getting 15mpg or so minimum. Had it since 1st April and never been above 3000 rpm! Was lucky to break over 10mpg in my ‘99 skyline! It’s all about how you drive it!
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Old 05-29-2019, 08:23 AM   #18
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Re: Yet Another Gas Mileage Question

Here is where I would start:
Timing. Set your all in and get your vacuum advance set. All in should be around 36*, vacuum is usually around 10-12*. You want to be sure your advance is coming in fast enough. If you are cruising 2500, but you aren't all in until 3000, you will not have enough advance. For vacuum advance, the real test is a cold engine, choke on, light throttle, through the rev range. When the choke is on, the engine is running rich, so you need less advance. My setup is somewhere around 16* base + 18* mechanical (34 total), all in at 2500 RPM, + 12* vacuum. You need optimal advance when cruising. When I am on the highway, I am pushing about 46* timing.

Next would be fuel. You have EFI so you can tune this exactly where you want it. At cruise, around 17:1 AFR should be best for MPG. Play around with leaning it out at low throttle / high vacuum.

Another thought is your gear ratios.
Calculating for 255/55R18 tires and a 0.70 gear ratio, here would be your RPM at 55 mph with different rear ratios
3.08 - 1,375 RPM
3.73 - 1,666 RPM
4.10 - 1,827 RPM

Do you have lockup on your transmission / torque converter?
I'm wondering if you are running it outside a range it is happy.
I wonder if there is a way you could hook your HEI into the fuel injection to get computer controlled spark. That would certainly allow you to optimize your advance at 55 mph.
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Old 05-30-2019, 12:58 PM   #19
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Re: Yet Another Gas Mileage Question

This rant is about Octane....I got all stock V8 307 that was swapped in sometime in the past form the V6 that it originally had, the truck is all stock how it came of the factory floor except the swapped out engine. It also has a three on a tree manual transmission.
So I would like to talk about the Octane as it was an eye opener for me to say the least.
On octane 87 which here in Ontario, Canada has 10% ethanol i would get a solid 10 mpg, about a month ago I switched over to 94 which here has no ethanol at all i got a solid 14 mpg, I know that you will say that 94 is more expensive but for us the 94 is only 20% more than 87 but with 94 I get 40% more mpg.
Now i am experimenting with Costco 91 as here in Canada it is also ethanol free and it is only about 6% more expensive than 87 at a regular gas stations(Costco 87 is about 5% cheaper than most gas stations)
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Old 05-30-2019, 02:14 PM   #20
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Re: Yet Another Gas Mileage Question

The difference you saw is not from octane, it is from ethanol. 87 octane has more energy stored than 91 or 94. The advantage of the higher octanes, is the higher detonation resistance. The difference in energy in 87 vs 94 isn't that large though. The difference between gasoline and ethanol is huge. Ethanol has about 2/3 the energy of gasoline.

Either way, I am surprised you even saw a difference given you are using a carburetor. The amount of liquid injected should be based on load and RPM, not how much energy is stored in the fuel. EFI would be able to determine rich / lean and adjust automatically. However, maybe the power increase means you can run with less throttle and RPM.
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Old 05-31-2019, 08:14 AM   #21
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Re: Yet Another Gas Mileage Question

This is gettin deep!
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Old 05-31-2019, 09:37 AM   #22
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Re: Yet Another Gas Mileage Question

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My 350, 3 speed is getting 15mpg or so minimum. Had it since 1st April and never been above 3000 rpm! Was lucky to break over 10mpg in my ‘99 skyline! It’s all about how you drive it!
These are UK "imperial" gallons right? Our gallon is something like 17% smaller so your 15 MPG would equate to 12.45 using the smaller gallon ( if I am thinking correctly this early AM). -BA
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Old 05-31-2019, 10:41 AM   #23
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Re: Yet Another Gas Mileage Question

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These are UK "imperial" gallons right? Our gallon is something like 17% smaller so your 15 MPG would equate to 12.45 using the smaller gallon ( if I am thinking correctly this early AM). -BA
What you just said ->

My head. ->

Iím going 4.5 liters to a gallon
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Old 05-31-2019, 10:46 AM   #24
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Re: Yet Another Gas Mileage Question

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What you just said ->

My head. ->

Iím going 4.5 liters to a gallon
Our gallon is 3.78 Liters. So (4.5/3.78)-1=.19, so an Imperial Gallon is 19% more than a US Gallon.
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Old 05-31-2019, 12:55 PM   #25
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Re: Yet Another Gas Mileage Question

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Our gallon is 3.78 Liters. So (4.5/3.78)-1=.19, so an Imperial Gallon is 19% more than a US Gallon.
I said our gallon was ~17% smaller than Imp. Gal. It was easier for me to then multiply his 15 by .83 (close enough for this purpose). But I am no mathematician.
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