67-72 3/4 ton with good gas mileage

[pjmoreland]

I would think that a manual transmission with a 3.07 gear ratio is going to be the best combination for good gas mileage. Automatic transmissions are not as efficient as manual transmissions. I had a 6 cylinder 250 with a three speed manual transmission and a .79 overdrive unit that got 21 mpg on the highway. I can get 16 mpg with my 350/700R4 if I keep it under 65 mph.

The problem is that engines are most efficient when they are near their peak torque at wide open throttle, but wind resistance becomes a really big factor at higher speeds. Little engines get better gas mileage because they can operate closer to their peak at normal highway speeds. You'd have to drive extremely fast to get a bigger engine so that it is approaching its peak efficiency, but wind resistance becomes more of a factor and more than cancels out the benefits of running the engine at peak efficiency. Big engines are really only practical if you need the power for pulling heavy loads over mountain passes. I doubt there are too many people on this board that care about being practical though, me included.

[Sillyoldman]

Figure out how to make it run good on four cyliders?

[pjmoreland]

Isn't that what Ford does with their F-350?

[chevyfarmer]

Retrofit a "displacement on demand" system like the new v-8's are running, the 345 hp hemi's in the new dod%e charger is suppose to be 24 mpg.

[mitchfab]

Thats a thought with the Use of Hydro-Gen!

[mitchfab]

Bla Bla Bla! Retrofit might work but thats some big bucks.

[TP from Cntl PA]

I am hoping to get around 40 with mine................But it isn't exactly a bolt in deal as you can see below

[Project1970]

Originally Posted by mitchfab

Ok I got a buddy with a 97 chevy that gets 24 with a 5.7

Your buddy is full of it, or rarely exceeds 55mph. One or the other.

[FirstGen]

Originally Posted by mitchfab

Yea! I love a hard pullin big block when it pins you back in the seat and all you see is the sky. its time to hit the brakes and get in line for another pass .

On a recent trip to Austin and back, I got 19.7 mpg on the highway and I prolly make 100 horse more than that 402 makes...

SPECS:

364 CID, FI gen III small block
T56 trans
4.10 gears
29" tall drag radials

[Maximum Overdrive]

Originally Posted by mitchfab

Bla Bla Bla! Retrofit might work but thats some big bucks.

If you really want 20 mpg with a 402 then no matter what you are going to be spending some big bucks to achieve it. If it was so simple the factory would have done it and so would everyone else and their mother.

[mitchfab]

Originally Posted by FirstGen

On a recent trip to Austin and back, I got 19.7 mpg on the highway and I prolly make 100 horse more than that 402 makes...

SPECS:

364 CID, FI gen III small block
T56 trans
4.10 gears
29" tall drag radials

Yea Thats what Im gettin at. You got a fI small block I think it could be done with a tbi setup and a overdrive .Anyway a gen 4 tb bb intake will fit a 402 wont it?

[mitchfab]

Originally Posted by Maximum Overdrive

If you really want 20 mpg with a 402 then no matter what you are going to be spending some big bucks to achieve it. If it was so simple the factory would have done it and so would everyone else and their mother.

You dont think a stock fi bb with overdrive will get close to 20 in a 67-72 out of a say 91 1ton gm?

[mitchfab]

Originally Posted by Project1970

Your buddy is full of it, or rarely exceeds 55mph. One or the other.

That could be true. But it we all got Big Boots here in EastTexas

[mitchfab]

Originally Posted by TP from Cntl PA

I am hoping to get around 40 with mine................But it isn't exactly a bolt in deal as you can see below

Thats sounds good.You got a pic.

[FirstGen]

Originally Posted by mitchfab

Yea Thats what Im gettin at. You got a fI small block I think it could be done with a tbi setup and a overdrive .Anyway a gen 4 tb bb intake will fit a 402 wont it?

Nope. They are 13 degree heads. Totally different intake runner ports. Won't work. Best bet would be an ACCEL Gen 7 fi setup.

[Cat Fuzz]

Originally Posted by mitchfab

Anyone got a tbi smallblock that will get 20?

I had a 77 Monte Carlo with a 327, Carter 4 barrel, aluminum intake, dual exhaust and all emission equipment removed. It not only passed emissions, it passed beating the standard for a 2 year old car and it got 21 MPG. It must have had something to do with the 2.56 gears in the rear That same engine is now in my truck and it gets about 15mpg now.

[FirstGen]

Originally Posted by mitchfab

You dont think a stock fi bb with overdrive will get close to 20 in a 67-72 out of a say 91 1ton gm?

Nope, no way. TBI is a poor design. Sequential FI is much more effecient. TBI is basically an electronic carb. To get the kind of milage you want you will need a very effecient head design, a great ignition system, and a good amount of overdrive. My tranny has a .50 overdrive in 6th...

[69sixpackbee]

Originally Posted by mitchfab

These gas prices are bad! Lets all get together and make it better with a big block!

Bad prices? Hell, son, you're in TX. Try coming to California!

[OlyHeeebz]

What do you guys think swapping a 6.2L diesel in would do for you? I saw someone on this board has done it. With all that torque, you could put some tall gears in the back (3.07's or taller), with an overdrive tranny, you might come out doing pretty well on the mpg's.

If I had better knowledge of how to do the swap and a donor engine, i'd consider doing it.

[72 CustomCamper]

My 396 only gets about 4 mpg. The only time I got 20 mpg was when my 72 had an 84 6.2 diesel in it...


Chuck

[TP from Cntl PA]

Originally Posted by mitchfab

Thats sounds good.You got a pic.

It isn't done yet, but I do have it up and running...............Works darn good!

Front suspension is pushed to the limit though

Others used this same engine(6AT3.4) and have averaged 30-35 mpg on the highway WITHOUT an OD................I have OD and steeper rear end gears so it should do well.

[67_C-30]

Originally Posted by 67ChevyRedneck

Everyone is dead serious bro. My 2000 Z-71 with a 327 and fuel injection will get 18.3 highway if I'm easy on her. My 67 with 350/700R4/3.73 will get 16 highway. Even the brand spakin new chevy truck regular cab 2wd with the 4.3L fuel injected V-6 and a 5speed tranny is listed at 21mpg. 20 mpg out of a big block???? Fuel injection, fiberglass body, 6 speed, aluminum rims, skinny driver, empty bed.... and you shut it off and roll it down a hill for 5 of the 20 miles
I believe txfirefighter has said before that he has a 67-72(dont remember the year) with a straight six and 5speed (again, cant remember for sure) but I think he said he could get 21mpg with it??? I apologize if this is wrong, but my memory sucks.

Not that it matters or makes a difference, but your 2000 Z71 engine is a 325 cu in. (3.780" x 3.62)

But I agree that 20MPG with a BBC is very optimistic is a truck, even with EFI and OD. They get about 1/2 that in stock form even in newer model trucks.

[Blazer1970]

20 mpg with any big block in one of these trucks can not be done.

[68Stepbed]

Comp Cams 270 or 280 cam, finely tuned Q-jet carb, HEI ignition with MSD 6A box, 3.42 rear gears with an overdrive manual tranny, Ceramic coated exhaust manifolds (not headers), Flowmaster Delta Flow Mufflers with 2.5 inch pipes. You might get 18 or 19.

For the cost to get good mileage, you could leave it like it is and come out about the same.

I had a 90 3/4 ton with a TBI 350 and a 5 spd manual and 3.42 gears and got 20 on the highway and about 18 in the city.

Just my .02

[Palf70Step]

There is probably a way to get close to 20 out of a BB, but how much cash are you willing to spend. You are talking tranny, rear end changes, injection and exhaust...etc....So for 5 or 6 thousand...probably more your likely to save 20-30 dollars a week in gas (depending on how much you drive maybe a lot less). If you want to sink money into something for gas mileage, get another vehicle. To me the price differnce isn't worth it. I like my truck and for me to cget another vehicle to insure, licenses and maintain...plus the cost of the initial vehicle (or changes to mine) is not worth what benefits I will get from it. So I just pay the extra $30 a week in gas. Just my $.02.