Looking for the Unicorn... Better highway MPG out of a 78 FJ40

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C

cvan1971man

Joined
Nov 15, 2019
Threads
207
Messages
562
Location
Arlington, TX
Got a 78 Fj40
- 2F Engine
- Holley 2bbl
- 4 Spd
- 31x10.50' tires
- Stock ring & pinion
- Probably get 13-14 MPG currently with mostly highway speeds 65-75mph
- This is my son's truck, he does not have a lead foot

Just realized that the set screw in the vacuum advance was impinging the vacuum advance and hasn't been working for the past couple of years. Performance was probably hindered, especially at highway speed.

I have looked at swapping the 4 spd to a 5 spd and you guys said that it really isn't worth it. I was wondering if changing the ring and pinion to a different gearing would be an economical option?

What would i need to look at to make an educated decision regarding the ring and pinion. any other options to look at?

Am i barking up the wrong tree or is there a unicorn out there for getting better MPG.
 
The thing is shaped like a brick and uses 50+ year old tech. Likely barking up the wrong tree.

Swapping to FJ60 (3.70:1 ratio) thirds would drop your rpm and make for a better road driver.
 
cvan1971man said:
mostly highway speeds 65-75mph
- This is my son's truck, he does not have a lead foot
Click to expand...
giphy (6).gif
 
Drive under 60 … you’ll get better fuel economy

It’s a FSP

normal driving 14/15 highway (65-70) … when I baby it I get 16+ …. Drive it like a hybrid I got 18… but too annoying do :meh:
 
Yup, 2F engine, 55-60 mph seems to be the sweet spot on the highway.
 
cstav86 said:
The thing is shaped like a brick and uses 50+ year old tech. Likely barking up the wrong tree.

Swapping to FJ60 (3.70:1 ratio) thirds would drop your rpm and make for a better road driver.
Click to expand...

My 1980 40 has 3.70 diff gears stock. With 33” tires, I can drive 65 mph all day, and go to 75 if need be. But my mileage isn’t any better, 12-14 mpg. As @cstav86 says, your son has the wrong vehicle, if gas mileage is a concern.
 
Better MPG? Turbo diesel with an overdrive, 3:70’s and at least 33’s.

Getting 16 to 18 mpg with a 5.3 LM7 (LS), 4L60e OD automatic, 4:10’s and 33’s. Turns about 2000 rpm at 65 mph.

Cheap? Buy a truck that has already been built.
 
All the above. ^^^

2f, 3:70’s, 33’s and an aluminum body. I’m still south of 14mpg rolling down a hill. 55-65mph is my happy place. You might pick up a little better mileage with a Sniper installed. As mentioned above, it’s a brick on wheels.
 
One other note, as you are chasing 1 or 2 MPG's consider how much fuel you can buy for the mods you are making to pick up those couple MPG's.
Here's the math to back that up: At 150 miles per week at $4/gal fuel if you go from 14 to 16 mpg you save $278/year. It's going to take years at that rate to pay out any significant modification to get those 2 mpg's.
Buy a used small car, that's likely a better use of funds. Save the 40 for weekends.
 
you could chop and channel it? - but with the 2F tractor engine, there are limits. I switched to a Holley Sniper with an in tank fuel pump - I get around 14-15mpg at 55mph. Will it go faster, for sure, but for me that seems like a good place.
 
Chungas Revenge said:
All the above. ^^^

2f, 3:70’s, 33’s and an aluminum body. I’m still south of 14mpg rolling down a hill. 55-65mph is my happy place. You might pick up a little better mileage with a Sniper installed. As mentioned above, it’s a brick on wheels.
Click to expand...

I think the key is the speed in which we drive like you said. At 55 I’m getting passed by trucks but that’s where the good mileage is. Funny thing is if I drive 55 in my Tacoma I get the middle finger salute from everyone, grannies included. If I drive 55 in the ‘40 kids wave, dad’s give the thumbs up and wives wish they were married to me ( just kidding about the last part). Driving 55, my wife doesn’t even wish she was married to me.
 
fireflyr said:
I think the key is the speed in which we drive like you said. At 55 I’m getting passed by trucks but that’s where the good mileage is. Funny thing is if I drive 55 in my Tacoma I get the middle finger salute from everyone, grannies included. If I drive 55 in the ‘40 kids wave, dad’s give the thumbs up and wives wish they were married to me ( just kidding about the last part). Driving 55, my wife doesn’t even wish she was married to me.
Click to expand...

IMG_4893.gif


Twin…it’s like you are living my life (minus the Tacoma…I’ve got a tundra) no hate when they see I’m rolling in the cruiser.



As to the mpg, I’ve got a 4cyl turbo diesel cruiser. 5spd and larger tires. I turn 2250 rpms at 65mph.

For 50,000 miles and 6 years I’ve been tracking…

IMG_4894.webp


Now if I drove 55mph on a straight flat highway tank, I bet I could be at 25-26…maybe.


Or….i can drive my 2002 Jetta tdi wagon like Heinrich Von Arseloch on the autobahn screaming “get out of mine vay! I’m late for lederhosen fest 26!” And still get 42mpg.








Sorry for my German cultural appropriation. It’s a joke not meant to offend any actual real life Germans going to lederhosen in the past or future. In the future I will try not to be so culturally insensitive to everyday German culture of overly complex cars, weird outfits, brawtwurst holders in vehicles and silly accents.
 
cvan1971man said:
Got a 78 Fj40
- 2F Engine
- Holley 2bbl
- 4 Spd
- 31x10.50' tires
- Stock ring & pinion
- Probably get 13-14 MPG currently with mostly highway speeds 65-75mph
- This is my son's truck, he does not have a lead foot

Just realized that the set screw in the vacuum advance was impinging the vacuum advance and hasn't been working for the past couple of years. Performance was probably hindered, especially at highway speed.

I have looked at swapping the 4 spd to a 5 spd and you guys said that it really isn't worth it. I was wondering if changing the ring and pinion to a different gearing would be an economical option?

What would i need to look at to make an educated decision regarding the ring and pinion. any other options to look at?

Am i barking up the wrong tree or is there a unicorn out there for getting better MPG.
Click to expand...
Mine got 11 mpg most of the time and the 16 gallon tank was a handicap up here in Wyoming with some towns over a hundred miles apart.
I changed distributer, carb, installed a header and nothing changed. I had to replace the 2F with a 155F and nothing changes.
 
fireflyr said:
Better MPG? Turbo diesel with an overdrive, 3:70’s and at least 33’s.

Getting 16 to 18 mpg with a 5.3 LM7 (LS), 4L60e OD automatic, 4:10’s and 33’s. Turns about 2000 rpm at 65 mph.

Cheap? Buy a truck that has already been built.
Click to expand...
I put a H55F and transfer case and 35" tires and replaced the F motor with a R2.8 diesel. Have not got it done to see what my mileage will be.
 
It was built when the national speed limit was 55 and this altitude and a head wind I could not do the speed limit on I-80. I hope that changes with the new set up.
 
Swap in an H55F and split case. Find a 8/80+ rear axle for the parking brake.
That said, the cost of the parts alone will buy quite a LOT of fuel...
 
Last edited:
Tigerstripe40 said:
Swap in an H55F and split case. Find a 79+ rear axle for the parking brake.
That said, the cost of the parts alone will buy quite a LOT of fuel...
Click to expand...
The H55F and split case with 3:1 low range was more then I paid for the 40 new in 1978. The parking brake can still be used with the split case. If you have the 4 speed case and let the builder know they can build the TC to accept the parking brake. All the parts are off the shelf and still available.

The H55F requires a cross member that the 4 speed does not have. But I kept the mounts for the bell housing and the cross member under the transmission.

For me it was already in my rebuild plan to upgrade to a 5 speed and split transfer case behind a diesel.

There is a lot of support available for converting to the H55F.

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