1978 F100 with trasnplanted 1975 460 ci. and an Automatic transmission.
Besides show-trucks or something fresh off the lot,I've never seen so-little rust on a truck. I brought it Home and my Wife liked it till I raised the hood...then all Hell broke loose. She enjoys the performance,as do I,but it is KILLIN' us just to drive it around town. It's fast...but like,Quick-Fast. Maybe it was run at the local Drag Strip, (Bristol's Thunder Valley) I'm not sure...but one thing I am sure of,is that it will damn-near pull the front end off the grond once the rear wheels,(P235-60-R-15's) quit spinning and smoking.
It has Carb. issues,as the floats are sticking,vacume lines are "plugged-off" with what looks like Screws and whittled-down sticks from a tree branch of some kind...but like I said,it's one quick-fast ride ! Then theres this issue of "topping out" thr RPM's too-fast for my taste. Example : At 70 mph it sounds like it's in the Red-Line-Zone somewhere around 7 or 8 Grand,I mean it's just SCREAMING !!! How do I correct this without spending a couple of House payments for a Trick rear-end ?
Nothing would please me,(and my Wife) more than squeezing out 10 mpg,(or more) from our new Toy...without breaking our already broke behinds anymore than I already have.
As a n00b here,I'm all ears concerning anything and everything involving the F100 Ranger and Big-Block Motors with Automatic Transmissions.
I bought this Truck in Honor of my recently departed Father,and plan it as a driving personal memorial to His memory. As I live in the Mountains of N-E Tennessee,I realize some torque is needed for these steep grades I drive on a daily basis,yet I also do a lot of Highway driving to and from work,(about 60 minutes both,there and back)...and spending $30.00 a day to go to and from work really pisses the Misses off. HELP me PLEASE ! I dont want to ever part with this Truck...but if I cant get better MPG out of it,She may Divorce me...then I would have to learn how to Cook and do Laundry too!

Tokor
p.s.
In case I cant get better MPG out of it,and She does leave me...does anyone know if you can Bake in a Microwave ???

 

Welcome to FTE! Honestly, if you're worried about gas mileage, a 460 cubic inch engine in a full size pickup truck is not for you. A tuneup and being really careful about how you drive might help you get 8 or 9, but that's pushing it. If you want the joy and experience of these old trucks, extra gas money is a responsibility you just have to accept. If the gas money is causing personal problems for you, then purchasing a smaller vehicle is a personal choice you're going to have to make.

 

Toker,
Welcome to FTE. There are a lot of folks here that can and will give you more specific technical info than I. However, first thing I would suggest is to keep a lighter foot on the accelerator and under 70mph. Your mileage drops significantly every 5mph or so over 55-60mph or so. Mine sure does! Also find out what the rear end is, fix the carb issues and the vacuum lines back to spec. Get a timing light and find out what is going on there. Not knowing what is stock and what is not may hinder your quest, but I do know that our trucks do not get good mileage. 10mpg is not unreasonable to hope for, but I think that best I have ever gotten is around 12.5mpg empty. 8.5 with a 4,000 lb camper on the back. 6 is ridiculous! As for the roaring RPMs, out trucks are loud, but I don't believe you are actually up at 7 or 8 RPM!! Is your transmission slipping though?
sorry I couldn't be more technical in my answer, but good luck!

Dak

 

I must have missed the part about you missing vacuum lines - is your vacuum advance hooked up on the distributor? That can help a little bit too. You mentioned the floats are sticking - how do you know? If the floats are sticking then the carb will flood out and it's not going to run at all (or barely). Can you be more specific?

 

If your spending 30 a day in fuel , your best to pick up a 4 cyl. commuter car for work and leave the truck for weekend / pleasure use.

Check to see what your rear gearing actually is . Make sure the vehicle is secure !!! Rotate the rear wheel with a chalk marked driveshaft and count how many revolutions the DS turns . I'd be curious to hear what kind of gears you have , if you suspect it might have been drag raced you might have high 4's in the rear. Or grab some codes off the tag , thats only accurate if the stock gears are inside though.

Might be worth your while if the low gears are in there ,to replace the rear end with something more highway worthy . 3.7's or 3.5 range . Would sure help your fuel mileage too.

You have any pictures of the truck ? I'm a big fan of 78's .

 

Heh, for a F100, I would be tempted to say 3:1 for a commuter. With the small tires he is running, at 80MPH he will be turning 3,000 RPM. If switching out the third member for different gears is out of the question, an upgrade to taller tires will bring down the RPMs, and tame the engines tendancy to spin the tires.

What I have to say pretty much mirrors what others have already said.
You are asking for poor mileage with jackrabbit starts, and spinning the tires. If you slow down, and simulate driving the truck like it has no power, you probably will see less fuel consumption. If you drive in a hilly area, don't expect great mileage anyways, its the nature of the beast. If you want fuel economy buy a Honda. You will be surprised how much money you save in fuel and maintenance costs if you stopped driving the truck to commute in. The money you save in fuel can pay for other priorities, and pay to fix up your truck to go get groceries in, go racing Saturday night, and to cruise on Sunday on your way to church.

 

You can always try one of these...


I've got a 460 in my F-150, 4x4. I've never seen better than 10 MPG.

Like the others have said, do what you can to get the best mileage out of the engine. Make sure it's tuned correctly. After that, higher (numerically lower) gears in the rear may help out. But the best suggestion I can come up with is to find yourself a small and cheap commuter car. Just weigh the cost of driving the F100 w/460 versus the cost of a small fuel efficient car, come up with a reasonable number and that's how much you can spend on a small car.

 

welcome to FTE!

basically the main points have been said.

review:
if the gears aren't in the 3:1-4:1 for a commuter truck then yes you're not gonna do good. find out what the gears are. if you need to swap go for 3.5:1 to 3.9:1.

your tires are only 27-28 inches tall. if you step that up your gas mileage will increase.

spinning the tires and knowing you have power. if you do these things then you're not gonna get decent gas mileage at all.

drive slower. I'm not saying drive 40.........fyi, at 55 you are gaining 2 mpgs over 65. just think about it.

fix the carb and vacuum lines.

all of these things will eventually get you better gas mileage.

 

Yes you can Bake in a microwave and if it was my wife or my truck she knows which way the door opens to get out. Actually easier to do without her than my truck ! I have 75 4X4 F250 with a 460 in it the most I have ever got is 12MPG empty in town it's somewhere around 5 or 6 depends on how far that pedal goes down when I take off ! I love when all 4 tires smoke when I take off and then let off and the exhaust just cracks. Nothing like the deep throaty sound of a big block you can't get that sound out of a small block.

 

Okay, time for my 2 cents. one of the most important items has already been mentioned...3.73's gears with stock size tires.
2. 2 words....gear vendor.

 

LOL :) Yeah and/or taller tires, 235'85-16's = ~31.5"

Tune-up!

192*F Robert Shaw thermostat.
If it's got a 180*F save it for a spare part.
If it's got a 160*F give it to your enemy. ;)

Get under the hood, fix everything you can find wrong and re-do
some old stuff just because it ain't new ...like ignition wires.

Then at least you'll know the gasoline ain't going out your tailpipe,
it's going out through your tires? ;)

Alvin in AZ

 

but what if I told you I have a web link to a former gear vendor technician who rebuilds and sells these things complete for about 25% of retail? Also, I speak from first hand experience...I just got done doing 2 of these upgrades...one on a 1979 Bronco and the other on a 1989 F350...end results are impressive.

 

I will post the link...I've had and heard nothjing but good things about this guy.

 

Tires are only 25" tall . I run the same size tire and run 2500 rpm at 60mph with a 302 and a 4 speed with 3.70 gears , I use to run a 275 60 15 28" tall and ran 2500 rpm at 68 mph , I would run a taller tire , they make a 235 70 15 at 28" tall which I will run next time , good luck , they dont call them big blocks for nothing , big gas , big power , good luck and welcome , lets see some pics .

 

I have always wanted a gear vendor on my truck. As Ben notes though, the price has always prohibited me from getting one. HOWEVER... for 25% of the original price??!! WOW!