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Old 02-07-2018, 09:23 PM   #1
Jarvis-69 GMC 1500CK
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Best fuel efficiency

Hello!

I am trying to decide where I want to go with my truck (69 GMC 1500 longbed) (I want 25-30 MPG with 2-300 HP, nothing crazy I am for sure going to be putting in a 5-6 speed manual which ever option i go with)

Here are my options that I am toying with - One of them might get me shot...but oh well!

Option #1 (I think would be the easiest).
Keeping my 80's 305 in it, Efi'ing it, head swap for a vortec etc... tuning it and really squeezing it for all it has (I do not wish to swap to a 350, if I do a swap it is to one of the other 2 options). If I do this what is the max I could see, again 5-6 speed manual and taller rear end gearing

Option#2 modest power diesel swap - just throw away the 305 and just put a diesel in it, if so again what numbers would I see with 5-6 speed

Option #3...this is the one that may get me shot and is kinda fantacy
2Jz swap (straight 6 boosted toyota power plant from a supra)

I have looked online and I see a lot of talk but I cannot find alot of numbers, especially for #1, that is the one I am most likely to do if I can get that 25-30 with EFI and head swap - etc

Also about 80% of my driving is highway so that 25-30 MPG number I desire is highway, I am not silly enough to expect that as city for even combined!

Last edited by Jarvis-69 GMC 1500CK; 02-07-2018 at 09:29 PM.
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Old 02-07-2018, 09:44 PM   #2
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Re: Best fuel efficiency

OK, I'll be the first to say it - the only way you're going to get 25-30 MPG out of a 69 GMC truck is if you coast it downhill in neutral. It's a brick, and it just doesn't move through the air very efficiently. Modern trucks struggle to get there with computer controlled engines and transmissions and bodies that are tested in wind tunnels. Just my humble opinion.
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Old 02-07-2018, 10:22 PM   #3
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Re: Best fuel efficiency

You can probably squeeze low 20's out of it. Slam it to the ground, air dam, 225 70 15 tires all the way around. Get the high pressure rated ones and run 44# air pressure. Keep open diff in the rear and get a 4.8 LS motor from a late model truck.

Weight is fairly low on these trucks, lower rolling resistance, keeping turbulent air off the bottom. Effiecient, smaller displacement engine. I'd just fine one that has the 6 speed auto trans behind it.
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Old 02-07-2018, 10:23 PM   #4
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Re: Best fuel efficiency

That gutless 305 won't gain you anything no matter how much you try to modify it. That truck weighs 4,500 pounds and you'll have to put your foot in it to make it move. More cubes will gain you better fuel economy as you don't have to mash the gas every time to move the rig.

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I would never rebuild a 305.
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Old 02-07-2018, 10:23 PM   #5
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Re: Best fuel efficiency

If you need 25-30 mpg, buy a used Honda and enjoy the truck on weekends.
I have an LS swap and can get 20mpg, but nowhere near 30...

Good luck!
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Old 02-07-2018, 11:06 PM   #6
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Re: Best fuel efficiency

If you go 45 tops, you might be able to get 30 mpg but not with a motor tuned for 2-300 HP. Two tons of aerodynamic brick rapidly builds drag as the speed increases. Plus all that HP mostly isn't used for highway cruising and is wasted efficiency. A small 6 or 4 optimized for low end torque is what you want but your acceleration will be on par with a loaded semi.

For comparison, one of my hunting buddies had a Toyota Tundra with a 305 sized motor that would get 22~24 at highway speeds but that truck came with all of the modern aerodynamics and computerized gizmos.
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Old 02-07-2018, 11:27 PM   #7
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Re: Best fuel efficiency

The only way to get close to that MPG is with a Diesel engine.
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Old 02-08-2018, 01:08 AM   #8
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Re: Best fuel efficiency

I think that mileage exceeds the capabilities of any SBC, in a vehicle like your truck. Now, take the Vortec 350 in my dad's '97 Suburban that would get 20-21 packed with 5 people and luggage on long trips and put it in a vehicle light and small enough, then maybe - but not in a truck.

I'm not familiar with the Toyota engine but I think you could potentially get those kind of numbers with the right diesel... but it ain't gonna have 300 horsepower. I want to put a Cummins 6AT (3.4L Onan design) in a 1/2 ton Suburban someday.
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Old 02-08-2018, 08:52 AM   #9
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Re: Best fuel efficiency

you'll never get 25+ mpg from one of these trucks. V8 swapped s10s get that kind of mileage, not c10s

option 1- no way. don't waste any money on a 305.

option 2-best chance at most mpg

option 3- know nothing about them, but doubtful it would beat the diesel.

if I was working on a budget, I'd go 6.2 diesel with some type of OD trans- nv4500 most likely. probably get you around 20 mpg.
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Old 02-08-2018, 11:48 AM   #10
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Re: Best fuel efficiency

Option #1 - You might hit 20 with a small LS or 267 small block. Easiest cheapest swap

Option#2 - I am thinking a Mercedes OM 646 out of a sprinter van might come close. You should do it!

Option #3 - Not a chance of hitting 25-30 with that motor in a truck. Very expensive swap
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Old 02-08-2018, 03:44 PM   #11
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Re: Best fuel efficiency

A stock 6.2 diesel backed by a 5/6 speed would probably get you 20-25. That would be nowhere near the HP you are seeking though. You simply do not have the aerodynamics to support that mileage.
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Old 02-08-2018, 06:49 PM   #12
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Re: Best fuel efficiency

I have a 69 GMC long bed, and if I see anything north of 12 to 13 mpg, I'm extremely happy.

With the original 307 it would get 8 to 10. I built a mild small block 400, and put in it and can get 12, and sometimes 13 on a really good day.

The 307 was straining to push a 4500 pound truck with the aerodynamics of a shipping container down the road at 65 mph, where the 400 isn't even breathing hard, so it uses less fuel.

300 HP and 30 MPG is near impossible for one of these trucks, no matter what engine /transmission you use.
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Old 02-08-2018, 07:03 PM   #13
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Re: Best fuel efficiency

Sounds like you'll be spending thousands to save hundreds. I second the econocar idea.
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Old 02-08-2018, 07:30 PM   #14
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Re: Best fuel efficiency

Here's an idea for you. Get a Vortec 4200 i6 out of a Trailblazer and back it with a 5/6 speed of your choice. Like said above, hard street tires with lots of air to decrease rolling resistance. Low stance and an air dam would help, too. Those late model straight six's don't get much love. There's a member that is local to me that is pitting one in his AD panel wagon. I loved the idea of a modern straight six cylinder in an old truck. Would probably be as fuel efficient as they come, without jumping into a 4bt, or something similar. And, they make better than 1HP/cubic inch, which isn't bad.

http://www.superchevy.com/how-to/837...ne-six-engine/

As for my trucks, I've never even CHECKED the fuel economy, but I am sure that I'm almost exclusively under 10mpg. My '59 is probably capable of better with its 350 and 700r4, but I drive that like I stole it, so yeah.

Also, you said that option #1 was to KEEP your current 305. I assume that it's in the truck now? If so, what fuel economy are you currently seeing? You could probably research what kind of MPG increases guys are seeing with just the Vortec head swap (remember that when swapping to Vortec heads, you will also need to buy a carbureted Vortec intake manifold. Then you can figure out what kind of MPG increases guys are seeing going from (whatever transmission you currently have) to a t56/Richmond/whatever you're interested in. Would not look to a 305 to solvensure any problems, except for maybe NOT spending money, if it's what you already have.

Good luck!

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Old 02-08-2018, 08:56 PM   #15
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Re: Best fuel efficiency

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Originally Posted by Dead Parrot View Post
If you go 45 tops, you might be able to get 30 mpg but not with a motor tuned for 2-300 HP. Two tons of aerodynamic brick rapidly builds drag as the speed increases. Plus all that HP mostly isn't used for highway cruising and is wasted efficiency. A small 6 or 4 optimized for low end torque is what you want but your acceleration will be on par with a loaded semi.

For comparison, one of my hunting buddies had a Toyota Tundra with a 305 sized motor that would get 22~24 at highway speeds but that truck came with all of the modern aerodynamics and computerized gizmos.
I drive a 2017 Tundra with a 5.7 and 6 speed trans at work everyday and the best I can get is 16 mpg.
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Old 02-09-2018, 03:58 PM   #16
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Re: Best fuel efficiency

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For comparison, one of my hunting buddies had a Toyota Tundra with a 305 sized motor that would get 22~24 at highway speeds but that truck came with all of the modern aerodynamics and computerized gizmos.
Tundra had a 4.7, which is more like a 283, or a 5.7 which is about the same as a 350. Seeing as how the 4.7 was rated at 13/16 for city/highway, I question the 22-24 number.
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Old 02-09-2018, 04:37 PM   #17
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Re: Best fuel efficiency

There are no v8 tundras getting 20mpg.
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Old 02-09-2018, 04:57 PM   #18
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Re: Best fuel efficiency

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Tundra had a 4.7, which is more like a 283, or a 5.7 which is about the same as a 350. Seeing as how the 4.7 was rated at 13/16 for city/highway, I question the 22-24 number.
I don't do the filling or check his results but I did look at the pump amount and it consistently took less then 4 gallons for an 85 mile trip. This was his first one with the approx 5 liter motor. His new one with the bigger motor dropped to 17~18. The station where we normally gas up on the way to the deer woods is flat ground so it should be taking a full load. He normally drives on the slow side of 70 so that could be the difference between what others are getting or 100% gas vs E10.
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Old 02-09-2018, 06:55 PM   #19
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Re: Best fuel efficiency

I have an 08 tundra with the 5.7 and when first got it, it got great mileage. Never officially checked it but did a long trip and think it averaged close to 20. Then it seems over nite it dropped. Now I get bout 300 to a tank, but it's lifted and has 35s. Tow package with 4.30 gears.
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Old 02-09-2018, 07:58 PM   #20
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Re: Best fuel efficiency

I think your best option will be a turbo diesel swap for better MPG.

I have been please with the 12-15 MPG my 56 gets. The 15 comes with conservative driving on a long highway trip. I think I can squeeze a few more MPG converting to EFI, but for now very please with what I am getting.

Good luck with your decision. Post back how it goes.
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Old 02-09-2018, 09:41 PM   #21
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Re: Best fuel efficiency

I did an S-10 frame swap on my old '53 GMC. I had gotten a really good deal on a complete running & driving S10 with a 4-banger and auto transmission. The engine had been freshened up and the transmission rebuilt, with all receipts. I built the body mounts and had carefully laid all of the wiring on top of the little 4-banger. I was going to see how good of fuel economy I could get with a low powered 4 cylinder cruiser. I bet that it would have been 20mpg or better! That truck weighed a lot less than it did as an S-10. I even cut down an old straight six GMC valve cover and made a bracket over the top of the engine, to give the engine the only cool looking touch that I could think of. When selling body parts off of the truck, I found people that wanted both the engine and transmission, so I scrapped the 4 cylinder econo-rod project and dropped a 350/4l60 into the truck. I kind of regret not building the economy truck now. Would have gotten twice the fuel economy that anything that I drive now gets. Driving a gutless turd is just hard to swallow. Plus it felt a little sacrilegious. Good luck on your quest for economy! Don't give up on your goal's just because a bunch of guys say it can't happen! You might be the first one to figure it out. I just HIGHLY doubt that it will happen with a 305!
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Old 02-09-2018, 09:51 PM   #22
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Re: Best fuel efficiency

You might try one of these:
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Old 02-10-2018, 08:22 AM   #23
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Re: Best fuel efficiency

My Dad has a 2-3 year old C10 or whatever they call them nowadays that knocks down 17 mpg at 1600 rpm rolling 70 mph down the highway. You have to use fuzzy math to get 20 mpg out of a truck.
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Old 02-10-2018, 10:26 AM   #24
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Re: Best fuel efficiency

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That gutless 305 won't gain you anything no matter how much you try to modify it. That truck weighs 4,500 pounds and you'll have to put your foot in it to make it move. More cubes will gain you better fuel economy as you don't have to mash the gas every time to move the rig.

Gary
I'm not trying to pick on you by any stretch, BUT....
I don't buy this at all. We've all probaby heard this rationalization for years. But, when I look at the most fuel efficient vehicles available, all of these automotive engineers seem to be going in the opposite direction. I think that your assertion is just not accurate. Though, I agree that the 305 is not the answer, but not because it's gutless. It's not efficient. Every vehicle that I've owned that got good fuel economy WAS gutless! It takes more energy to spin a big V8 crankshaft and 8 rods and pistons. I have a Quick Fuel Q-Series 850 carb that says that just because you're pushing the skinny pedal 1/4 the way down, instead of 1/2, it doesn't mean that you're using less fuel. Every V8 I've seen has had a significantly more robust fuel delivery system than any 4-banger I've had. The argument of big engines getting better fuel economy because they use less gas has always seemed ridiculous to me. Doesn't add up. If new semi truck's are going to be mandated to get 7 MPG loaded, this dude can get 30 MPG out of his C10! He will just need to think outside of the box to accomplish it. 10 years ago, I'm sure lots of semi truck drivers would have said that it's not possible to get 7 MPG Out of a loaded big rig. I talking out of my @$$ now, with semi truck fuel consumption. I had heard on the news about new regulations for these tractors/trailers.

I had a 1985 Toyota Hilux pickup as my first truck in high school that was a diesel. It was a factory diesel with a Mitsubishi (I think) 2.5l in line 4 cylinder diesel. I remember at one point in High School budgeting $5 per WEEK for fuel! Granted, I worked at an airport and the aviation fuel delivery truck driver always gave me that last 5-15 gallons of JetA fuel out of the lines in the fuel truck! I dumped a little bit of Marvel Mystery Oil in it for lubrication. That truck got insane fuel economy! OP, good luck on your quest! Work on reducing weight and wind resistance, too. Look the stuff that guys are doing in Bonneville to their street cars to increase speed. The same modifications will help fuel efficiency.
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Old 02-10-2018, 12:14 PM   #25
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Re: Best fuel efficiency

Ok.....so you've heard from a very knowledgeable group....and you may have to abandon the 25-30 mpg dream
However....here is a question, why do you want to get that mileage?.....
If you are commuting a lot, get a vehicle that is sized and built for better mileage.

All good
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