20 mpg or not?

[mrhino]

Within the week or so, the wife & I will be starting the engine build for the crew cab project we have going on. Over the weekend, It brought up a interesting debate with a few buddies that I trust and respect for their engine performance knowledge. My Goal with this engine is to get 20 MPG. A couple said it could be done and a couple said it wouldn't happen. Everyone made their points and theories. The debate and conversation was educational and interesting so I thought Id ask it here and see what some of the thoughts are.

The truck is a 88 1 ton crew.
4:11 rear
305/70/16 tires
700r4 trans

the engine will be a .30 over 5.7
58cc 187 swirl port heads with a mild port job.
20cc dished pistons
vortec Roller Cam 191º/196º 0.414"/.428" lift on a 111º LSA.
1 5/8 headers

so do you think it will get 20 MPG?

[thotfulspot]

Since our vehicles have the aerodynamics of a brick I would be surprised if 20 mpg is possible unless coasting downhills.
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[Keith Seymore]

I suspect no.

I can get 20-22 ish with my truck, which is a 1/2 ton shortbox with a 4.3, 3.73 gears and overdrive.

You've got your work cut out for you, especially with those 4.11 gears.

K

[Restrorob]

Originally Posted by Keith Seymore

I suspect no.

My 6 cyl 2x Jeep doesn't even do that.....

[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.

[mrhino]

I always new it would be a tall order. we all need goals right? I Figured I would be at about 1900 rpm at 65 mph

[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.

[piecesparts]

Originally Posted by Keith Seymore

I suspect no.

I can get 20-22 ish with my truck, which is a 1/2 ton shortbox with a 4.3, 3.73 gears and overdrive.

You've got your work cut out for you, especially with those 4.11 gears.

K

Where do you drive to get that good of mileage? city/ highway/what speeds/etc...?

[Keith Seymore]

Originally Posted by piecesparts

Where do you drive to get that good of mileage? city/ highway/what speeds/etc...?

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

[SSC's76]

the engine cam and head combo is capable of doing it but like others have said you need to overcome the weight of the truck. i would suspect you will see 10 and 14 out of that combo in that truck. I would personally choose a more tq oriented camshaft it would provide much better results with your rear gear and trans your working with. anything that makes power from idle to 4500 would be great depending on your cruise rpm if it made max tq 500 + or - in that range thats where you will see the best results

[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.

[GM-power]

I highly doubt it. Its hard enough to get that with a sbc in a half ton.
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[57rookie]

I work for the worlds largest heavy truck manufacture and we own our own engine company as well. My experience tells me with the current rear axle ratio, the rolling mass of the one ton, and the engine displacement with the specs you listed that 20mpg on a true highway only average is not realistic.
I would suspect if you watch your speed (60mph max) you might achieve a true highway only average of 17 - 18 mpg.

[bohmankid]

If you can get that beast to get 20 mpg, you should be an award winning engineer.
The weight, aerodynamics, size of the motor, tires, and wheel base all say no.

[ivorton74]

pretty ambitious but like the others here I don't think u would achieve that. you would be better off swapping a 4.8 or 5.3 LS motor. you'll probably get 18mpg + with the LS motor...or switch to diesel.

[king-918]

you're thinking about it all wrong, rid yourself of the old technology. time to find a hybrid Tahoe and get a Harvard grad

[KQQL IT]

6bt will do 20+
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[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.

[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.

[meatwagon83]

not 20, maybe 16 or 17 if you keep you foot out of it

[Oilbrnr]

Yes, if idling off of a cliff into a mile deep crevasse.

Otherwise, no.

[74 stepside]

With a Gearvendor added to that 700R4 you might get close, but realistically I dont see it. Heavy truck plus its lifted, thats a plow in the wind. No aerodynamics.

[D13]

My 87 R10 gets 23mpg mixed, would get 26+ at 55 on the highway. But I'm running a 6.2 diesel, no turbo. SLOW.

[tucsonjwt]

For reference, the best I got out of a V8 was 18.5 highway on a 2010 4.8 GMC extended cab with 3.23 gear. On a 2010 V6 LWB Silveado I got 20 city/24 highway, but very little power. For gas mileage I think you should consider an economy car, and drive the truck sparingly.

[camok10]

I was averaging 16mpg out of my k5. I went from an average of 10-12 stock with 273's and 32's to a 4" lift a 44hd and a 14ff with 410's on 35's. it was just a mild tbi 350 with a low end cam, intake, high flow cat with dual 2 1/2" straight pipes off it.

I don't see 20, but I'd imagine around 15 possible out of it
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