79 bronco, 400, 4 speed, 37 12.5 doing a 3/4 ton swap, Going with 14 bolt in rear. keeping the D44 in the front.
The 14 bolts has 3.73s
D44 has 3.50s

Would you change the D44 to 3.73s
or the 14 bolt to 3.50s?

 

With 37's and 3.50 axles you should be turning about 1,987 RPM (say 2 grand for giggles). Thats a little low for most V8's if you are running without overdrive. ('79 400's are rated at 159 HP @ 3800 RPM and 276 ft./lbs. @ 2000 RPM). The midpoint between those two is 2900 RPM which is a hefty jump to make if you suddenly need the power (climbing hills, passing traffic). With the 3.73's you would be turning closer to 2100 RPM which is still a jump but not as far to go and will still not kill you too badly in the mileage department... well no worse than running a 400 CID V8 will already kill you. If those are the options, the 3.73's would be my suggestion based on this. With 37's I'd be looking at taller still and maybe considering a way of getting OD somehow.

The one good thing about the 400 is its the longest stroke V8 Ford ever made so the power is there if you can dig it out from all the limitations that early emissions tuning and (relatively) low compression pistons and heads left it.

 

I don't have a tach, but feel like there is a lot of power still left above 60mph, got poped at 88mph (still had some left) going up hill about 3 months ago.

 

Yeah its there I'm sure. I have no real world experience with this setup so I have to go by the numbers.

 

If i had to pick between the two i would go with the 3.73s, but i would slao be saving my pennies for a future regearing of both axles.

 

I would honestly run at minimum 4.10's for that tall of tire. remmeber your not only turning the tall tire but also the added weight of a taller/larger tire. both will be a power drain. also remember the np205 tcase doesnt have that great of a low range so when you do need the gearing you will be lacking.

 

Dana 60 with 4.10s.. but if your turning 37s 3.73 is good but it wont be enough

 

The 3.50 seam to be doing great now? Dont know that I need to go down to 4.10s

 

I am not an expert on the 400 4spd.

My Bronco has 3.55 and 35s, E4OD
My excursion has 4.56s and 37s, 4R100. It had 3.73s which were awesome with 35s and not towing. They sucked when towing so I regeared it. It is more fun now but sucks more gas.

My guess is that 3.73 will do fine when not towing or needing that low gear. I would not do the 3.50s

 

I gotta agree with him.. 3.73s are good for highway and enough for light towing..
But I do have a 93 bronco with 3.55 gears.. e4od.. good enough for 35s but not for towing
I have an 04 f250 4r100 and 4.10s.. this thing will pull anything.. its a monster
4.56 has to be bad on gas.. lol

 

At 33x12.50 tires, I would like to go to 4.10 gears in mine, but I have 5th gear for the highway. 5.0 is lacking some torque for pulling loads but does fine down the highway unless driving into 40 mph headwind. At 37" I would definately want 4.10 gears. will make getting the correct tooth count easier on the speedo gear.
If it's a budget problem go with the lower gear set, and save up for even lower if in the future you feel it needs them.

 

I always think its good to check a gear ratio calculator, and compare it to the power band of your engine(the 400 makes it power down low). how your using the truck is also a huge factor. mudding, crawling, and working all need different gearing, not to mention a daily driver. here's a calculator i use.

RICHMOND Street Performance Calculator

 

I guess im used to trucks with low axle gears because there would be no way i would run 3.73 with 35s on the street permanently. I want deeper gears now and i have 4.10s with 33s

 

If you opt to consider gearing OTHER than the two listed in your initial post, then I agree with udsuth's suggestion to keep the power band of the the engine in mind when making the determination about gears. The 400 has the longest stroke of any Ford V8 mill so its low-end is definitely stronger than many of the others. If you are going to do a lot of highway miles without an overdrive gear and you get too tall you will kill your mileage. On the other hand, if you are going to be dragging around heavy stuff or climbing over obstacles or up hills, having that torque multiplied by taller gears on the bottom end will make light work of things. So, I guess the best answer is, "Research the engine and base your decision on the intended use of the truck."

Use this formula on any calculator or the calculator in your computer:

MPH x Axle Gear Ratio x 336 / Actual Tire Diameter

 

grey is absolutly right. purpose and power decide your gear ratio. that t-18 does give you a little more wiggle room, but in the end "purpose" is the deciding factor.