I have a 77 Ford F-150 Truck. I wanted to get some feedback about thirdmembers and to find out if changing it will help my fuel mileage. I now have 350 gears, What gear ratio would I need to go to to get better fuel mileage? As of right now I can pass anything but a gas station. lol
Motor is a M400 with a C-6 transmission.

 

Welcome to FTE, you will find a lot of great info on here.
well first thing what size tires do you have, what rpm's are you cruising at?
basically usually 3.50 gears with stock tires are not too far off of about as good as it gets, the 400 is not exactly known for being a economical engine adn the aerodynamic brick of a truck doesn't help either (but they are the best looking trucks around)

 

Put in a 2.75 rear, it will improve your gas mileage but your low end torque will suffer somewhat. I have 3.70's in my 6 cyl but will be putting in 2.75's in a few weeks.
Randy B

 

Welcome to FTE

1973/79 F100/150 9 inch axle ratios available: 2.75-1 / 3.00-1 / 3.25-1 / 3.50-1 / 3.70-1 / 4.11-1.

The 1973/79 Ford passenger cars also offered the same ratios, except in 1978/79 when a 2.47-1 ratio was offered on LTD's; LTD II's; and T-Birds.

 

Thanks for the respone's on the different gear ratio's. What i am trying to figure out is, Witch one should i use? This is what i am looking for, also what my equipment is. I own a 77 f150. My motor is a m400 2 barrell holly, c6 auto , 350 gear's, and my tire's are 15x60. What my intension's are is, to better my fuel millage. I do not intend to pull or haul a great deal of wieght. All that i realy pull is a john boat that will wieght at about 200lbs with all or equipment when going duck hunting. My last long trip in my truck, i only went 250 mile's round trip. And the gas it took to do this trip was out of this world. The amonut of gas was 30 gal's. I realy was not driving stupid. I was only traveling 60 mph. So if any one could please help me with this! Please reply.... thks nea1redneck...................

 

Since you're not using the truck except for recreation, I'd opt for the 2.75's.

 

my 77 with 351W, C4, 2.75:1 @65 MPh gets 15.5 MPG

 

Iguess i'll just stay nwith what i have. I do believe that i will have to live with8 mile's per gallon. That or go into depth with a newer truck. THANKS TO ALL THAT HAD REPLYED.

 

That engine is not known for good gas mileage but 8 is a little tight. It should do a couple miles per gallon better. Have you done a tune up lately? You might try running you timing up a couple of degrees.

you could repost in the engine forums and address the mpg issue. There are a lot of knowledgeable guys that might have some good suggestions for bringing that mileage up.

 

you should get 12. I got that out of a bronco with 31x10.5 tires- 3.5 gears.

An important consideration is what rpm are you running when at hwy speed...that will better determine which gears you want. 15x60 tires will really have the engine humming I'd think. All engines have individual operating ranges where they perform the best with the least amount of fuel. Your job is to figure out how to get it running at optimum which could get you 2-4 mpg's and then where to operate it at. As an example a friend has a boat with a 400hp diesel...at 1600rpm he runs 8 knotts and flows 5.9 gal/hr...damn good. at 1900rpm he runs 9 knotts at 11.5 gal/hr and at 2200 rpm he runs 10knotts at 15 gal/hr. Guess where he runs???

All equiptment is the same...ya gotta figure out where the rpm needs to be cause if your even a bit off it can cost you dearly. You need differrent gears...bottom line... and the difference in $ parts will pay for themselves then some.

I'd never settle for 12 let alone 8...the 400 can do a little better than the wheezer imo....i'm currently working on both and am not convinced either way.... but for best mileage the 400 has to be built right and a stock build post '71 is no where near right and engine builders seldom know how to build the 400 right. But that takes $$ which could be used buying gas yada yada.

But you get that engine running right/solid and get the 275 gears and you will be happier no doubt...better yet keep the 3.5 and get a gear vender over/under drive so you get a final drive of 2.7 and can keep the 3.5 for when needed and you can keep the engine in the 'zone' far better with 6 gears. Thats what i'm doing with all my 'keeper rigs'.

another example:
We were getting 21mpg hwy in wifes p.o.s. 05 toy 4 runner (consistently)...i drove it on a road trip and used the manual shifting available with the 5 speed auto..It's geared too high in a lot of situations and could tell using cruise...having the pedal respond etc...started downshifting to 4th even with cruise engaged to keep rpm maximized...got 24.5 over the 1000k trip and it took nearly half the trip to get it figured out.

You will need a vacuum gauge in the cab to learn more about how you need to drive...the toy had the mileage computer which worked ok....i calculated mileage using math not the computer...miles driven / gallons etc. I'd prefer a vacuum guage.

 

Would it be cheaper at least short term to spend that money on gas for what you have instead of a truck payment and gas for a newer truck and possible higher insurance??

 

HELLO ROGER DOWTY, I have read your reply. Thank you! I realy do not know what my rpm's are at 60 mph. Just by listening to the motor i can tell when it is about to top out. No i do not listen to the radio to where you can not hear your motor. I am a retired truck driver, I do now what a motor sound's like when it is about to top out. A complete tune up was done last week. Hell if i drive it more than 40 mile's a week i am doing to much driving at this time. After driving them big truck's for 24 year's i realy do not like to drive if i can help it.LOL. At a idle my motor run's at 8rpm's,timing is set acording to spect's,(Chilton)ALL fiter's changed, Plug's, oil, grease, breather valve, plug wire's, button and cap, and what ever needed repaired. YES you could say i take care of my truck as well as my wife's car. (ECLIPSE) All that i am trying to do is better my fuel milage. So i will tyr and see what i can do on finding a 2:75 gear's. Once again thank you...REDNECK

 

My 79 F150, 351M, C6 has the 2.75 rear and is just about right. These engines (351M and 400) have plenty of low end torque for the taller gears.

 

buzzman; What is your fuel milage with the 2:75 gear ratio?

 

Buzzman: What is your fuel miage? I need to know this imfo before i go and change the 3rd member.....