20 mpg or not?

[86c20]

well not saying you will get 20 but there are a few things that you need to do to help get it there. 1. gear it right. lot of guys over gear there trucks. find where your power band is and try to get your cruise speed in it. most stock small blocks are around1900ish to2300ish. not saying it is the most poweer but it is not luged down so much you have to shift alll the time. 2. air dam under the bumper. makes a big diff. my buddy did it on a dump truck and he gaind .25 mpg on averag. on a pickup i would think .5ish. 3. timing. timing is a big part or eco. in my c 20 the more retarded the wores i get. agian it is about freeing up power more then making power. 4.narrow tires. pizza cutters are the best. and free up all rolling resistanc. brake adj. should be right also. 5. elc fan. again freeing up power. did it in my monte ganied about .5 on the avg. 6. lock up trq convert. is better. standerd trans is best.

on the highway with my c20 i get 20's at 65-70 but my truck has stock size bfg at. and a crate350 that is 260hp and i run reg gas. good luck

[gchemist]

With a diesel, it's possible. LS engine with FI, very close. Old school carb, very unlikely.

My truck can do 18.5 MPG at 70 with a lot of work. It's only a SWB 1/2 ton.

[kalbert]

Not a chance. Even without doing any simple or fancy calculations I'm still pretty confident no. You might get close with a 5.3 swap, but even then I'm skeptical, those things barely get 20 new!

[Ziegelsteinfaust]

Originally Posted by kalbert

Not a chance. Even without doing any simple or fancy calculations I'm still pretty confident no. You might get close with a 5.3 swap, but even then I'm skeptical, those things barely get 20 new!

My 07 GMC rcsb with the 4.8 got 22hwy at 75, and I had the bolt ons with a tonneau cover. When I added 4.33 gears it dropped to 21, but the city went up to 15-16? I really don't know the city mileage but 17-18 is what I averaged rolling in morning traffic to a job for a few weeks. Also that with with almost no real hwy cruising.

My old 77 C10 rcsb got 21-22 hwy with a detailed 350 with a comp 260 cam, th350, and 2.73 gears. The tires were 245/60/15, and 255/70/15. I used a AC solenoid for cruise control, but it was set at 70mph. It had all the bolt on, but the big reason it worked as well as it did was the carb. I had it professionally built, and I kept tuning it till it was 100%.

Imagine what that 4.8 could do in a much lighter 77 with the less rotational weight of 15's. I knew a guy who was just finishing up his swap, and could was just about 25 hwy with 3.42 gears. Excellent yes, but what if he kept at it.

I think one of these trucks is capable of just about 30mpg hwy with a new motor like a 3700, 4200, or 4.8 vortec in a rcsb. Yes it is a brick, but these trucks were designed in a wind tunnel so there probably not that bad when you get right down to it. The weigh 3700 stock in rcsb form, and that could be trimmed down to 3400 or a bit less easily. With a lighter engine, and a few other detail things with out loss of comfort. The Colorado cc weighed 3800 pounds, and with the 5 cylinder could get above 25, and with all the bolt-on with tuning. Driven right my friend was at 30 hwy with his.

Personally I am done with new trucks, there cheap plastic interiors, and the bed sides that dent when you look at them. When I build my truck my goal is 20 hwy, but I am making mine a light duty C20. So I have a better margin of safety when I tow a camping trailer for when my boy gets older. It will be lowered as far as I can go with out cutting into the bed, tonneau cover, and some "modern" engine I haven't determined yet. I know I can do because my truck will weigh less the 3700 pounds, and be tweaked for my needs. My 07 GMC weighed 4750 on a truck scale, and dwarfed my slightly pre-runner F250.

[bohmankid]

Id be happy to get 12 mpg with my 5.3 swap!
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[74 stepside]

Just wondering if all of you that claim high mpg numbers, if you all had your speedo recalibrated to the change in tire diameters. Whether bigger or smaller, it makes a difference.

[piecesparts]

I spent a fair amount of time with the speedos on my 84 and 91, The 84 required some changes in the driven gear department and at 70 MPh it is only off about .5 MPH. The 91 had a computer DRAC module in it, that I replaced with a different one to get it close. It was off about 1.5 MPH. My 15 MPG in the 84 and the 19 to 20 in the DD 91 truck were as close as one can get and that includes the new vehicles, too. It is nice with the new trucks, the computer can be adjusted form the seat with a laptop program and a GPS to get them on track. I like to know what I am doing for speed, because some of our Law enforcement officers like to lock a high number in early and then write tickets off of that all day long. Most people will not challenge it. I also have a chart of what it takes to do a mile in the amount of time, for a variety of speeds and I can check my speedos whenever I want to, by using that and then compare them to the GPS. Not that I am accusing anyone special, he knows who he is.

[Keith Seymore]

Originally Posted by piecesparts

Not that I am accusing anyone special, he knows who he is.

Since you asked a couple follow up questions, and since I'm special (lol) I'll assume it was me.

Keep in mind my truck is the shortest, smallest, lightest, lowest (lowered even further) combination offered in the product portfolio, with the smallest (fuel injected) engine, smallest tires and manual overdrive trans.

But - your comment about speedo calibration has given me pause for thought, because although it is the "born with" factory setup I do find that 70 mph indicated is 67 mph per my GPS. That implies the odo could be off, to the fast side.

I wondered why everyone was always flying past me. I thought it was just because I am old.

K

[86c20]

i know my odom and speed are dead on. and i have stock size tires. my k30 on the other hand well it is off some lol

[Rollo McFlurry]

Not even close.
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[Low87]

I was getting 21 out of the '87. TBI & hugging the ground.

Good luck, I hope you can do it.

[KQQL IT]

A friend of says 22 out of a 4.8 variable displacement LS driving like an old man.
In a 1/2t x cab
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[D.FENS]

After all the talky talk, I say put it together and see if you can tune it enough to make the numbers you want.

It'll make for an interesting and informative build thread regardless.

[mrhino]

Originally Posted by D.FENS

After all the talky talk, I say put it together and see if you can tune it enough to make the numbers you want.

It'll make for an interesting and informative build thread regardless.

Definitely! The wife is going to be putting it together, so if it dont make the 20 it will be her fault.

Iv done the LS thing before in my 71. Maybe someday ill drop one into this truck, but for now im gonna see what I can do with the old school stuff.

Like I said before. I know it is asking alot, but i honestly figured Id have more optimism though. You guys are making logical points. I cant argue with them. in fact, I agree with most of them. But I either have high hopes or Im delusional. one of the two.

[piecesparts]

If you quesation your odometer, then time your trip down the highway. Use the mile markers,but do it on more than one mile marker. Actually the most effective way is to time a 5 mile stretch and then divide the time by the miles and then divide 3600 by the number of seconds to do the mile. You use 3600, because if you were going 60 MPH, it should take you 60 seconds to do a mile. 60X60=3600. Work form there for faster milles per hour.

[INSIDIOUS '86]

You need torque. The swirl ports will help and the porting was a good idea. What you should do as well is use a tpi system. A tpi injection is synonymous with torque with the small long runners and large plenum. This alone will make it way easier to achieve. You also need to find were your cruise rpm is and get a cam that will setup peak torque in that rpm range.
The 700r4 will help with getting that hulk moving and the lockup torque converter will be good for hwy mileage. Idk if you intend to use it as a full one ton is intended but less weight is equal to more power but without using extra gas to make it so that's a win win. Lighter susp and taller gears will help certainly.

I think that will easily net you 25mpg hwy avg
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[INSIDIOUS '86]

If you can get those heads and the cam and intake with tri y headers to all work together and pull hardest to 3000rpm then low gearing may not be necessary cause remember the 700r4 has a pretty low first gear.
Try what you have first but defiantly optimize the engine. If you have the money you can even get the ls computers to run the tpi and that way if you do get an ls engine you at least have that one part already.

The rear diff maybe you can swap to a suburban ten bolt with 3:73 if the ones you got are just to much for street. The suburban rears have bigger drums on them already and are way lighter overall but plenty strong for trucks still
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[mrhino]

Thanks to Insidious I revisted the Tuned port Idea and Iv decide to give it a shot. These old tuned port make a sh!t tons of torque ( well in this case a 1/4 sh!t ton) and combined with those old "junk" swirl port heads, which were only designed to make torque. Iv increased my chances of achieving my 20 mpg goal. I have also decided to use a diffident cam to better suite this combo. 196* 206* @.050 and 109 LSA .431 .451 with 1.5 rockers

[piecesparts]

Kind of like this?

[mrhino]

yes, but yours looks a little nicer. are you running it in a truck? what is your impression of it?

[INSIDIOUS '86]

Originally Posted by mrhino

Thanks to Insidious I revisted the Tuned port Idea and Iv decide to give it a shot. These old tuned port make a sh!t tons of torque ( well in this case a 1/4 sh!t ton) and combined with those old "junk" swirl port heads, which were only designed to make torque. Iv increased my chances of achieving my 20 mpg goal. I have also decided to use a diffident cam to better suite this combo. 196* 206* @.050 and 109 LSA .431 .451 with 1.5 rockers

Looks good. Doing the same thing but with a 283 with tbi heads and a tpi system. Just on a SLIGHTLY smaller k5...lol
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[MrBeast]

Originally Posted by mrhino

Thanks to Insidious I revisted the Tuned port Idea and Iv decide to give it a shot. These old tuned port make a sh!t tons of torque ( well in this case a 1/4 sh!t ton) and combined with those old "junk" swirl port heads, which were only designed to make torque. Iv increased my chances of achieving my 20 mpg goal. I have also decided to use a diffident cam to better suite this combo. 196* 206* @.050 and 109 LSA .431 .451 with 1.5 rockers

Running a TPI will help with the MPG's I would suggest looking into running a 411 ECM and sequential MPFI the TPI engines were all bank to bank, the sequential engines were much more economical.

[piecesparts]

Right now it is still hanging on a stand waiting for that special truck to go into.

[thotfulspot]

Off topic, but do you know what size bolts you are using to hold the block to your stand? I just bought a 6.0 and need to mount it to a new stand when I pick it up tomorrow.
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[MrBeast]

I would say you have a snowball's chance in H-E double hockey stick of getting 20mpg out of a 350 unless you have full EFI, custom tune, no emissions equipment and a 4L80E

Considering you are in a CC dually you are going to need 19.5's to bring the rolling resistance down, and lots of other things.

Really you would be better off tossing a Cummins in it.

I have a Vortec 454 in my 2000 CC Dually, it is much more aerodynamic than yours, everything on it has been tip top tuned, cats and emissions gear removed, running 65mph I get 14 mpg empty.

with a 350 you are going to find that with a 350 you are going to be underpowered in that dually which means less MPG's especially when you hook a trailer to it.