well 20 is my ultimate goal, but would it be safe to say that a gear change in back would be a good help in reaching that goal? Would 4.56 be a good improvment in most cases? or did some come with them stock?

 

No, in spite of what that chart said, you really want to stay away from something as low as 4.56. Those gears would be great for climbing trees or pulling stumps, but for gas mileage on the street, even with your 31s, you just wouldn't be happy.

Here's a link on decoding your door jamb sticker. The axle code should be on there, sometimes it's just not printed in exactly the right place.

https://www.ford-trucks.com/article/...r_Decoder.html

If you just can't find it and the tag is missing from the axle, you can jack up the rear end and turn one of the rear wheels by hand, counting the number of turns the driveshaft makes relative to the wheel. Put a chalk mark on the driveshaft for reference, and use the valve stem for your reference point on the tire. Turn the wheel exactly one turn while counting the number of turns the driveshaft makes. If it takes 3 3/4 turns of the driveshaft to make the wheel turn once, then that's probably a 3.73. If it's a little more that 4 turns, more like 4.10s.

You can get a more accurate number by getting a friend to help you. You have your friend turn the wheel exactly 10 times (slowly...unless you can count really fast!) while you count the turns of the driveshaft. If it takes 37 and a third turns, that's the 3.73. 41 turns would give you 4.10s...etc.

With an automatic trans stock in an F-100, you are probably more in the 3.00 range.

Hope this helps. I just gave myself a headache!

 

Well i got a 3 speed manual tranny. But i do agree that 4.56 is a pretty big gear. My door jam sticker really is devoid of axel codes so i may just jack it up at the rear end and count it like you said. But what would you suggest for a good gas mileage gear?

 

3.73 or so would be my opinion........ MAYBE even 4.11 depending on what speed you typically get the truck to.....

 

i usually drive between 30 and 50 city and 50 - 65 highway. Would 3.73 still give me some good towing? What about 4.11?

 

65 on the highway is a little much for 4.11's

4.11's at 65 with 31 inch tires would be 2895 RPM Thats kinda high

 

no, but with the 30 inch tires that my wifes truck has and its 3.00 GR and C6 final drive ratio of 1:1 I come up with a miserable 2016 RPM

 

You're running rich. Vacuum leaks will make it run lean, so that's not the problem (although you should fix any vacuum leaks you can find). A constantly open EGR would make it run lean as well so I doubt a problem there as well. I suspect that your carb may be leaking internally, so that's where I'd start. Does the rpm wander at idle?

 

no, and no. Sadly you're kinda late into this discussion, i have a brand new carb and manifold. It's nice to cruise in third gear now, although i do feel guilty driving at 1.1k RPM. I know swapping in the 4 speed will make a huge difference with the milage. Also, now that i have a job again i will purchase my MSD upgrade. 20 - 25mpg is my ultimate goal!

 

Sounds like that at 2.75 final drive (at least that's what door tag says), 2000 rpm at 60mph is high too. That's what you are saying right? And my rpm wanders like crazy at idle. So far, changed carbs, changed timing chain, changed distributor - but the rpm still jumps around. When I pulled the plugs out, they look nice and clean - no indication of running rich though. Although, I suspect that my engine has a higher duration cam. how do you find out what kind of cam is there? The engine was rebuilt by some guy and I cannot seem to trace him down.

 

It's overly complicated, but you could take the valve cover off and measue hour much the end of the pushrod moves for a aproximate measure of how high it is.