rear end ratio change 84 F250 6.9, 4x4
 

I have a 1984, F250 6.9 Diesel, 4x4 with a C6 automatic. I am trying to learn what it would take to change out the 4.10 rear end to something that would get better gas mileage, I'm getting about 15mpg at 60mph. The truck will mostly be used for light to medium duty. The differential has 70-18U cast into the top front. From what I had read I expected it to be a DANA 60 or 61, but the "70" stamp has me confused. The door tag says axle type "73". So nothing is matching.

What would be the lowest ratio (3.07?) I could go with the least amount of effort and money. I believe it would be a ring and pinion swap.

I am also trying to find out what it would take to put an AOD or similar transmission in it.

Thanks in advance

How are the tires? If you are due for a new set a size change will have
the same outcome as going a few points, BUT don't get crazy wide ones to reduce
friction and some of the weight. https://www.ford-trucks.com/calculators/index.php
Theres some calculators to figure what changes will have.
You will have to change the front gears also or leave your driveshaft
out and effectively have a 2wd. If you had the money for a overdrive
unit like one of these that might be the way to go. http://www.gearvendors.com/4x4.html You could split all your gears using
this if you wanted to!!

You need to also find out what front axle you have. I believe the dana 44 might be able to go down to a 3.07 or 3.08 ratio. But I believe it takes reverse cut gears in the front. If you could get the ratios front and back down to around the 3.0 ratio, you probably could get almost 20mpg. Otherwise, you might be able to only go down to a 3.54 ratio, which would be a improvement over the 4.10.

The only tranny that is going to hold up to a diesel is going to be a E4OD. The problem with it is it needs a computer to run it(they are available).

If you add up the rearend change + the tranny change, then it would be cheaper to buy a $2500-$3000 overdrive unit and bolt it on, and shorten the driveshaft.

Thank you for the replies and the information. I just got this truck for $1500, so I'm still learning about it. I got the differential numbers and called a specialist and found out I have a Dana 70 in the rear and and a Dana 40 in the front. The best ratio those 2 units can have is 3.54, and I fount out they already have it. So, I'm stuck without doing an axel swap. The Gear Vendor units look good, too bad they're pricey. The tires are in pretty good shape, but I will run thru the calculator to see what size would be an improvement.

Great information on this forum!