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

[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.

[Restrorob]

Originally Posted by Keith Seymore

I suspect no.

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

[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.

[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

[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.

[meatwagon83]

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

[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...?

[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.

[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

[mrhino]

I knew the weight of the truck would always be my downfall. Tire width was something I hadn't considered but it will add to the problem. Iv been playing with cam grinds. to find the best torque numbers. I mentioned the vortech cam cause it something iv had laying around and it it didnt seam too bad for bottom end torque and still be TBI friendly. 500 ftlb would be nice but im not sure I can do it with those heads. The engine is not together yet. so things can still be changed.

[rs72z]

I can get 18 on mostly highway with my ccsb dually. I have a 5.3, 4l60e and 4.10 gears. I doubt 20 will ever happen old school.
I would like to get mine out on a long tripp to see what it will do.

[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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[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