Just got a 91 F150 4x4, 4.9 l, M5od r2, 3.08 rear end and 31" tires. First gear is crazy tall...3.9 going into the 3.08 diff. Motor turns 1700 rpm at 60 in 4th gear (4th ratio is 1:1...5th gear is 0.8) Not trying to win any races going up hill with this one but looking for ideas about how to bring the ratio down a bit for starting out when pulling a load. Don't have a big trailer..yet. I'm building a tiny home on a 8' x 12' trailer for my teenager. Might be 4,000 lbs or more when I'm done and I'm worried about the clutch. Also would like to be able to tow 5,000 lbs occasionally for farm work. I know I could swap rear and front end bits (been told by a wrecking yard guy that this truck doesn't use a 3rd member). I found a 3.53 combo for about $500 and a few scraped knuckles. Wondering if smaller tires would work on the 15 inch rims maybe?

And saw another post here about starting in 4low and shifting to high while moving. Mine has auto hubs so not sure if that would matter.

 

First off, you cannot shift into or out of 4low while moving. You must be stopped and in neutral.

what you need is 4.10 gears in the differentials. Find an f250 4x4 (many came with 4.10) and swap that third member with yours. The d50 diff is a bolt in operation. That saves a chunk of change, only having to set the rear gears up.

 

I'm a little confused about terms...I've been told that this truck doesn't have a third member. I guess it uses some other set up to transfer power. Whatever the case I know the ratio has to be the same for front and back.

Is the d50 for the front?

Would the rear end from an F250 have a stronger axle, and 8 lugs instead of 6?

Sorry for the newb questions...I'm pretty confident with a wrench and youtube but have never messed with axel stuff before.

 

The front diff does have a 3rd member but the rear doesn't. 3.55 gears were very common in these 1/2 tons and 4.10 was an option but not that common, the easiest solution is to pull the front 3rd member and complete rear axle from a donor 1/2 ton truck(F150 or Bronco) and swap it in.

 

So sounds like I could bolt on a third member and swap the rear axle complete if the ratios are the same. What about tire size? I don't need the big 31 inch off road tires that are on it. I've seen some flatbeds with what look like much smaller diameter tires. Can you get shorter side wall road tires for trucks?

 

As noted, your front axle should be a D44 TTB independent suspension with a third member differential. The rear should be a ford (Sterling) 8.8. It is a fixed case differential with a rear inspection cover.

the D50 (or D44HD) came in place of the D44 in f250 pickups. All three third members will directly swap from 83 to 96/7. 80-82 will bolt in, but used a different method of retaining the passenger side axle shaft. If you find any of these TTB third members (D44, D44HD, D50) with a 4.09 gear set, it'll bolt on. I recommend doing the D50 stub shaft swap while you're in there. It adds a bigger u-joint and yokes in the passenger axle shaft. I have a thread from another website if you are interested.

from there, you move to the rear. Either find a half ton or bronco with 4.10 gears and swap axle assemblies, or buy a new 4.10 gear and pinion and have them installed.

the f250/350 rear had an 8 lug setup, but will bolt in place. It is a sterling 10.25 and is quite strong. However, then you need two spares. Or convert the front to 8 lug using Chevy parts. The Dana TTB arms are vastly different between f150 (D44) to f250 (D44HD or D50 and is a major hassle to swap. It involves cutting and swapping crossmembers and such. The last option would be just find an f350 and swap both solid axles in. That's quite a swap for the un-inclined however.

keep the tires. Otherwise you'll have to reprogram the speedometer. They look better too.

 

Super helpful reply, Bigblue. And please do post the link you referred to. About the tires...I don't care about the speedometer being accurate, I drive like my grandpa...lol! And as for vanity, I cut my hair once a year whether I need it or not. Is there any point in pursuing smaller diameter tires?

 

Stock tire size for most of these trucks was 235/75R15, just under 29" tall. If you'll be towing but not off road I'd make the switch. I know my truck did much better on fuel and street performance with them over bigger tires. This would make 3.55 more reasonable ratio, enabling you to save some cash (because it's unlikely you'll locate a 4.10 equipped 8.8" rear). If you keep the 31" tires, going to 4.10 would be your best bet.

 

With a 91, not changing the Speedo will be fine. 92+ it would affect the trans.

This is the thread i followed when i tore my whole TTB down, and did the stub shaft swap

https://www.fullsizebronco.com/forum...k-warning.html

 

On a 91 all you have to do to make the speedometer read correctly is change the speedometer gear on the end of the speedometer cable.

 

Lovin the feedback everyone! after thinking this through a bit more I'm leaning toward gears instead of tires. True I could gain a few rpm with smaller tires but the impact wouldn't justify the expense. For about the same amount of money it seems like I can find 3.55 rear axle and third member. Or maybe just by new ring and pinion gears. From what I read the process is delicate but none of the videos I've watched look too hard. I have a decently appointed shop to work in and time...it's my farm truck, not a commuter.

On a related note... I finally found some time to pull the plugs and they look baaaaad! No sign of fouling but very old and used up. Putting new plugs, wires, cap and rotor in this weekend. Hopefully that will help with low end power. Did I say...my big concern with gearing is first gear. Truck goes 8 mph at 1000 rpm in first. I'm most worried about starting out on a hill with a load. Don't imagine anything more than 3-4000 lbs max, but with 3.08 rear end first gear is really a dog.

And did a compression check on 5 cylinders (#6 is blocked by a bunch of wires and vacuum lines tied to a bracket but hope to get to that one tonight). all from 130 to 142 psi on a cold motor. Planning to run a can of seafoam through the intake sometime in the next week as well. I took off the plastic air intake hose that mates up with the upper intake manifold and the inside of the manifold is very clean...no carbon residue.

 

3.55 or 4.10 gears will help you on hills in first. Instead of 8 mph, you'll be doing like 6mph at 1000rpm.

my bronco does 8mph at about 4000
rpms in first with the granny low and 4.56 gears

 

And looks like 33's on the rims,too.

 

my calculation puts it at 4.8 mph in 1st at 1000 rpm's for a 4.10 rear end and 6.6 mph for 3.55 rear end.

btw...any idea if it's possible to change the 1st gear in the m5od to something a little lower?