20 mpg or not?

[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.

[piecesparts]

Originally Posted by Keith Seymore

Roughly 30 mile commute (one way); mostly expressway, as fast as traffic will allow (my goal is 73 mph, probably ends up at 65 mph or so on average).

K

I agree with the others, the possibility of getting high mileage is usually in our own minds. My 84 GMC standard cab,SWB, has a 385 stroker engine with a 3.73 gearing and a highly modified 700 R4 tranny. I do not have a lockup converter, but I do run a 2400 stall to keep the cam under control. The smaller tire diameters allows me to run around 2300 to 2500 RPMs at 65 to 70 MPH. I get 15 MPG on the highway and a lot less if I am on city streets working the engine. My old daily driver 91 Silverado with a 5.7, 700R4 tranny and 3.42 gears got 19 to 20 MPG daily and repeatedly as I drove it for the better part of 10 years. It was also a SWB truck with a standard cab. The fact that you have a serious amount of weight is against you and the gearing and tires sizes also work against you. You need to keep the RPMs in the power band and not TOO LOW or TOO HIGH. That is where you look at gearing the tire diameters for optimizing the performance.

I am looking at your combination and my thoughts are NO, you will not get 20 MPG, you will have to be creative on getting the parasitic loads off of the engine and then it can maximize what you do get. Electric fans will gain mileage, the intake and exhaust setup wiill help in this, get the most spark that you can to the engine, low resistance spark plug wires and a multi-spark setup to get a complete burn of every ounce of fuel in the cylinders. Then look at tires and widths of tires for the best style and lack of rollling resistance for your approach to a mileage gain.

[hatzie]

Originally Posted by Ziegelsteinfaust

I don't think you get past 17 with those tires. There to wide, and heavy for mpg.

Plus you 14 bolt is a heavy girl at 550 pounds, and you'll need to do the disk conversion for rotational weight reductions. The disk conversion brings your 14 bolt down to a slim 475 pounds roughly.

Plus I think a 455 Buick will put down the torque you need for those tires if you need them, and the short stroke you need for mpg. I had a bolt-on "except headers" Electra for a minute with one, and 2.73 gears. It would regularly get 19mpg cruising 70 with the cruise one. I have a spare sitting in my garage from when I got the car, and I am on the fence of putting it in my C20.

Agreed. The tires are way too wide for good fuel economy.
At least 3.73 rather than 4.11 would be the other change to make.

The big 455 Buick or 472 Caddy engines are torque monsters and could comfortably cruise at lower RPMs' than a Smallblock.

[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!

[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.

[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.

[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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[wklink]

Originally Posted by jitteryguy

Certainly will have to change the gear ratio, the 700R4 will help with a lower ratio perhaps evan lower than 3.73. I'll be curious to see what you do get.

I was about to say, not with that rear end.

[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.

Originally Posted by 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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I have been seriously considering the TPI. like you said they are great for torque. Years ago I built a sweet Iroc with a very nice 5.0 TPI. Everything was proffesional ported and matched, plenum, runners, intake, heads, block, even the TRI Y headers. That thing was a beast. it ran 13.6 in the 1/4 and would get 28 mpg on the highway running 75-80.

Originally Posted by wklink

I was about to say, not with that rear end.

I went back and forth on what gears to run. I had 3.73 available to me if I wanted them. In the end I decided to keep the 4.11. Here is my thinking. The truck will spend a good portion of time driving my wife back and forth to work. Through town with lots of stops and turns. I figured since she is a big girl as a couple of you had pointed out.( the truck, not my wife she reads these posts from time to time) The shorter gear would aid in getting those big tires and girth of the truck moving much much easier and would result in less fuel being burned, at least through town. Yes it sacrifices fuel mileage on long freeway trips. but the taller gears make it much more difficult to turn those tall tires and get her up to speed. Gears are a trade off its one of those things where you cant have your cake and eat it to. For me there were really only 2 logical choices for gears. 3.73 or 4.11 I chose the ones I thought would do the best considering where the truck would spend much of is time. Maybe 3.73 would have been a better choice IDK.

[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?

[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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