hey gang. My 07 f150 screw 4.6 2wd does a fair amount of towing. It wasn't bought to do it, more a matter of circumstances. So my pinion seal is puking lube, I have high miles, and no low end power. I currently have a 3.55 ratio. I'm putting a 3.73 in. I'm not looking for major difference, I realize it's not much of a change. But I'm not putting 3.55 back in.
so once I change the gearset, how do I deal with the computer so speedo is right? Ford? Or are there other options?

 

3.73s alone won’t up your tow rating enough to matter. It may give you a few hundred lbs, akin to swapping out the 18s for 17s, but that’s a matter of tire ratings anyway. I think my old 4.6 ‘08 SCrew was rated 5900 lbs with 18s, 6400 with the beefier 17” wheels and tires.

You really need to swap in a 9.75” rear to help things out as that’s got a higher weight rating. But, that’s relative, since you still have a high revver 4.6 doing the work for you.

A tuner will deal with the speedometer calibration issue.

 

I realize its not a big change. My truck goes decent down the road, the takeoff is the issue. I am building a truck that replace this one so long term towing isn't a problem. But I'm about take a 2000 mile round trip, my pinion is leaking, I assume bearing is toast, so I bought complete bearing kit decided to try the 3.73. I don't care if I lose a little speed. Its runs like 2100 at 80mph. I have plenty of room on top. It takes off like a golf cart. I'm hoping to gain just a little bit on take off. If I don't, then I don't. I have do this trip in a little over a week. I didn't have time to order parts after I dig in so bought everything to do the job. I have 326,000 miles. No clue about maintenance. Chances are that it needs what I'm doing. Even if no gain in power it will at least be good for a long time after this
Main concern was that I wasnt sure if I had to go to ford or not for change in ratio.. I'm happy to hear that a tuner can do it.. thanks for letting me know!

 

as for towing capacity, I have 17's and rating is 6400. My trailer is rated at 7000. I'm not trying up capacity. I probably don't normally hit the 7000 limit, but if I do it doesnt bother me any. But with 6000 behind me, starting from sitting still, its far from impressive. Makes me wonder if I stop on a hill, could I get going or would I be backing up. Lol
hope the ratio change gives me just a little better takeoff.

It will have to do until my Cummins F2 is running

 

3.55 to 3.73 is nothing. I'd be surprised if you notice anything, but there may well be a placebo effect.

Be sure to break in the new gears properly, first 500 mi no towing and short trips with full cool downs ONLY. Yukon has instructions on their site.

 

I understand that there is a break in procedure. I worked on 18 wheelers for years, when I replaced a diff in anything, the break in it got was by normal use right out of the box. Nobody has time to fiddle around for 500 miles. Right or wrong, this thing is going to get a little local driving for a week or so, and then it is likely going to be hauling a trailer 1000 miles. Done similar with other vehicles, no problem. May not be the way engineers prefer, or those ford techs want per the manual, but it works!
I went to diesel tech school, studied drive trains, I get the theory, but it comes down to reality.
The speed changes with this ratio are minimal but there. At 2100 rpm it changes speed in OD by 4 miles per hour from 80.45 to 76.57. 1st gear at 1000rpm from 9.44mph tp 8.99. Subtle yes. I didn't want to go all the way to 4.10. I found a 3.90 option but I decided just to go with 3.73. Without a load the truck goes pretty decent, I didn't want to make a tractor out of it. I am building an F2 on a superduty chassis with a 5.9 cummins. That will be my tow rig. If i didn't have mechanical problems with the diff I would have left it alone, but it opened the door, so here we go. lol

 

So I forgot to post my thoughts after the gear swap. So I put a Richmond gear set in and all new bearings. Ratio change from 3.55 to 3.73.
I did the install, drove the truck around town for 2 days, then hauled a ford expedition 1000 miles one way, some around town and highway for 2 weeks and hauled a ford focus 1000 miles back. Since that trip, just local driving.
the ratio change wasn't much so I didn't have big expectations, but after putting some miles on, I do have this to say. I think this ratio is what ford should have put with the 4.6 engine. this truck may have slightly gained a little more pull from this change, but what the ratio change really did that i think improved it the most was change the rpm at a certain speed just little but in a way where the truck likes it much better. The gearing this way seems better for the driving I do most. And it's not much different but the engine rpm, road speed and transmission gears just work more in tune. Before, it was just on the edge of downshifting at certain speeds where now its just enough that it doesn't have to. On the top end, its a little slower, but not enough to worry about as this truck was really geared fast anyhow. I think it was running like 80mph at like 2100rpm before, or something along those lines. Same rpm now is about 5mph slower. No big deal. But while pulling a trailer, what I think the benefit is,, is with rpm, speed and gear selection. It has just more rpm in a gear at speed than it did before which with the 4.6 already underpowered didn't mind. It just seems to help most by that little rpm bump.
all that said, would I do it again? yes, but if I really wanted to gain bigger pulling advantage I'd go with a 3.90 or a 4.10 gear set.. in my case,, i don't intend to tow with this a lot longer so i was shooting for a little boost while not drastically changing fuel economy or drivability. My truck has 330,000 miles and I had a job for it. So at those miles, with leaking pinion seal, I decided to replace gears and bearings because I didn't have time to take it apart and then wait for gears if I needed them. I just got everything shipped here so I had it all when I dug into it.
I'm happy with the results, I like the shift points better now, I think although its a small difference, it works better this way. If I was planning on doing more long hauls I would definitely recommend a bigger ratio change as actual towing isn't much different. Really this is a crappy truck to tow much with as the 4.6 just doesn't have enough power, but as I said before, I didn't ever intend to tow with it when I bought it, things just worked out that way. Its holding up great though and I really have no complaints about it. I just run it hard when I have to.
I have a diesel super duty that will be replacing it shortly so its towing days are numbered.
I wouldn't change to this gear ratio just for the sake of doing it, but if you have a worn set of 3.55s and are in there changing stuff anyways, its certainly not a bad idea, I feel like it is an improvement, even though it's small. Just worth it to me for how the transmission works better.
Oh and fuel economy is nearly identical.
thats my 2 cents on it. Happy tinkering!