Just weighed my truck

[Racer Z]

Now that is something to think about..................... Im eyeballing my friends 1970 f100 short bed w/ 390 c6, hes wanting to sell it, so maybe that will be my daily driver, at least im thinking it will be cheaper on gas. .....

It might be lighter, but I bet it's not cheaper on gas.

Although, an engine conversion would cure that. A new motor in an older truck. Hmm.

My brother has new Jeep SUV that we use sometimes to tow the race car. Sometimes we use my 70 F250 w/390. With all other conditions being the same, I get about 9 mpg and he gets about 15 mpg. Empty I get about 14 and he gets about 25.

[chepdog1]

He said his 70 short bed gets about 12/13 mpg on freeway, hes got 4.11 gears so i'd swap them out for 3.54 asap, and throw in a 4 bbl carb along with a nice matching intake (im pretty easy on the gas and have heard mixed reports about gas mileage doing this. (I used to have a 1975 highboy 390 2bbl 4.11, motor was in excellent shape but gas mileage was horrid, it got maybe 6-9 on a good day and didnt have the punch of a 4bbl ) a nice fuel injected 302 with a 4 speed auto would be nice on that f100.

[PhantomoftheBumpside]

Hmmm.. 390FE 2v + C6 + 4.11 = 13mpg/HWY? I doubt it. AND 3.54 is a Dana 60 set, 4.11 is Ford (4.10 is Dana). Which rear does this thing really have and what is the code? G 17 or G A1 (auto/heavy ford/3.25) would probably be about right if really getting 13...

[Racer Z]

There are many variables that do effect fuel consumption. Without knowing all the specifics it's not a fair comparison.

I have no idea what's in by brother's Jeep other than he bought it new and it's all stock. I'm pretty sure he has a V8. He has plenty of power to maintain speed up the hills.

I've got a fresh 390 with Carter 625 CFM 4v and Edlebrock Preformer 390 intake manifold. The rear end is a Dana 60, but I don't know the ratio, probably a higher ratio. Tires are 9.50x16.5 steel belted radials. Stock exhaust manifolds with a single tailpipe. I currently have a camper shell, but will be putting my lumber rack back on soon. I bet the MPG drops then. Tranny is a C6 of unknown origin (could be out of a car). When I'm pulling the racer I have a full load of fuel, 53 gallons which is about 350 pounds. I also have tools, spare parts and other goodies.

[chepdog1]

Hey Phantom im not sure what rear end hes got but he definately knows hes got 4.11 or i guess 4.10 i would have to talk to him again, plus hes got a locker in the rear ? according to him, he says that when he makes a "u" turn his rear tires are slightly barking. I would change those gears to more usable city and highway use. Maybe swap a 302 with a 4 speed auto and drop some 22's on it, paint it hot rod black and be done with it, oh yeah and a split bench seat from a newer Ford or Chevy.

[PhantomoftheBumpside]

There are many variables that do effect fuel consumption. Without knowing all the specifics it's not a fair comparison.
...
The rear end is a Dana 60, but I don't know the ratio, probably a higher ratio.
...

3.54 (C7) and 3.73 (C8) are pretty common Z...

Chep... If the rear is original, the last part of the 2nd line of the warranty tag in the door will tell you... "lockers" are a Letter and a number... "open" is two numbers... 17 is 'open' Ford 3.25 ... A1 is the 'locker' Ford 3.25 C7 and C8 are locker Dana 60's...

How many people have you run into that think they have 4.11's because they have a locker, because "that's what a locker always is" whenever they read the specs in their favorite 'car' magazine?

General rule... 4.11's and up suck gas... mid 3.'s are 'reasonable' and low 3's and under 'sip' gas... of course that is still all relative to the shape, size, weight and power plant of the given vehicle. Throw in an overdrive or torque-splitter in the middle and things change a bit.

[chepdog1]

I will make sure i get some kind of picture or numbers of that rear end that sounded funny (pictures of the rear end) and post them here so you guys can see the rear end. I think its time for bed.

[Ripsnorter]

I've always been curious what the MAX MPG would be that these old trucks could achieve.

They're not astoundingly aerodynamic, but then not even new trucks are.

I thought about putting something like 3.5xs in my truck, with the ZF5 overdrive it should do okay. Then possibly something like a Fuel Injected 5.0? I wonder what the best combination would be.

I still have this belief rattling around the back of my head that if a person could build a 460 with the focus on being as free breathing as possible (as little restrictions as possible, porting, etc all done) and a decent set of gears, manual tranny with overdrive, maybe even a gear vendors on top of that, so that when you're cruising down the freeway you're only spinning 1500 RPMs or so. A 460 has more than enough power and low end grunt, it shouldn't have any problem down at that low of RPM.

C6 Trannys lose just too much power, (I've heard the figure thrown around about 60 HP lost just to a C6, but I don't know for certain) with a manual you'll gain there as well.

So, perfect world, empty 5,000lb truck, easy breathing 460 putting along at 1500RPM at 70 MPH... Why couldn't a person achieve close to 20MPG?

Seems to me everyone with a 460 focuses on big gears and offroading. Thing is it has enough power to idle down the freeway, but the power is still there when you need it, especially if you had a 5 speed or better, PLUS an under/over drive unit behind that. 10 speeds and a 460 you should never have to worry about power in the normal line of work.

---- Edit -----

RPM Calculator says that a 3.73 with ZF5 and Gear Vendors with 32 inch tires will be spinning 1600RPM at 70MPH.

Just my thoughts and ponderings.

[chepdog1]

I know its gettin kinda off the original subject, but thats what id like to do also, a stout 302 or mild 351W with a 4 speed auto with some low 3.?? gears, i figure the f100 short bed probably weighs under 4000lbs, so id say close to 20 mpg with a good setup is not a hard feat, ........................ (manual tranny is ok but i like the freedom of just stepping on the gas.

[averagef250]

1600 is way too low. It just isn't efficient down there regardless of engine size. A 460 won't push a truck 70 at 1600.

Forget dumping loads of cash into parts and machinework to make a gas engine do something it can't. Buy any one of a growing number of acceptable turbodiesel engines for much less money and drop it in. You get lots of power and double, even triple the mileage.

[Ripsnorter]

Our Pontiac Bonneville cruised at 70 MPH at 2100RPM. No lack of power. When it was needed it just dropped it down a gear or 2. And that was with an automatic and a 3.8L

My little BMW 5 speed can idle down a road at as low as 1000 RPM and be at well over 40MPG without lugging. Granted I can't do that on a hill, but on the flat I can drop the RPM way down without problems. It's favorite cruising RPM is 2000.

[Racer Z]

Our trucks have the aerodynamics of a parachute. Well, all trucks do. Cars can get better MPG because they are inherently more aerodynamic, even if it's not intended.

To get good fuel mileage you must stay in the power band of whatever engine you have. Two grand comes to mind as the start of the power band for most older V8's When you get out of your power band (higher or lower), your using more fuel to make the power needed to push the air out of your way and pull the weight of you truck.

Over all gearing is important too. Tire size effects this a lot and most people ignore it. Just like small gears (4.11:1) will hurt you mileage, so will big gears. Why and how? Out of your power band.
With small gears your spinning the motor more and going slower. More RPM mean more gas used.
With tall gears, the motor spins slower and you go faster. This is all good until the RPM drops to low and you're no longer making torque. To make low RPM torque you need to tune the motor to do that. RV/Towing type cam is the first thing. An RV/Towing type intake manifold. Long tube exhaust headers. Yes, I would use a four barrel carb, probably a slightly smaller venturi than what most would consider using. Low RPM means low air flow which means small venturi.

This is quite the opposite approach that most talk about. Most want to set their truck or car up for what I call drag racing. Low end torque is gone and the motor only makes power at high RPM. Short tube headers, high-rise manifold and a big venturi works well here. But at the cost of using LOTS of fuel.

[70shortbox]

My reg card says 3580 unladen.

[Ripsnorter]

Racer, I realize that not all engines can putt along at 1500 RPM and still move a big heavy truck at 70MPH. A little 20 HP lawn mower, obviously not. I honestly believe the 460 has a low enough power band that it can manage it though. It would depend obviously, on how the engine were built. Just like you said about the cam, exhaust, etc.

Obviously a 302 that wants higher RPM is going to suck when lugged down low. A 300 I6 on the other hand, loves the low RPM.

My BMW will putt along at 1000 RPM until about 30 MPH. Then I can run it at 1500 or thereabouts all the way up to 55 when I don't have the gearing anymore and it revs up. I can't always climb hills at that speed, but I can climb some. I shift around 2500 or so, contrary to most guys that shift even up to 4000. And I get around 30 to 35MPG with it average, compared to the 25 to 28 it's supposed to get.

As you said though, most people do the opposite. Big engine, low gears for rock crawling, mudding etc. I just think it's possible to go a low RPM route instead. in my experiences with a 460, I do believe 1500 at 70MPH is feasible without the engine lugging and gasping. One of the big advantages of a 460 has always been bottom end torque and power without much work, why not use it?

[Racer Z]

Rip, I don't know the particulars of the 460, but with big displacement comes more torque. Even if the 460 is out of it's power band at 1500, it might have enough power simply because of it's massive displacement to do what you want. Better MPG from lower RPM is definitely the right track.

The Dana 60 was used by all the truck manufacturers for the 3/4 ton class trucks of this era. Not all of them use the same gearing as Ford. Check into the Dodge's. I think they had very high gears. You might get lucky and find the spring pads are in the same place. If not, spring pads are easy to relocate.