So I've got this 95 F150 with a locked up 3.31 rear end. I've found a guy with a rebuilt 3.55 rear end for an excellent price. My only other option at this point is to swap with another 95 F150 with the exact same rear end, except it's on a 4x4 and has a rear sway bar.

I guess my first question is what is the difference between the 3.31 rear end and the 3.55?

Second question, would it be wise to put the 3.55 in in place of the 3.31?

Third question, which rear end would get me the best gas mileage out of my truck?

Eng and Trans are 5.0L with a 4r70w.

 

1) The difference between the 3.31 and 3.55 is a ratio of 0.24 which means if you're high way RPMs were 1,600 with 3.31 gears, they'll now be 1,716 RPM at the same speed.

2) If you have a 4x2 truck, I'd say do it. I don't believe the rear axles and mounts are any different between the 2wd and 4wd trucks (unlike the front), so that shouldn't pose any problems. Also, buying an already rebuilt rear end is probably going to be cheaper (and easier), than messing with rebuilding yours.

3) I've got little experience with the 5.0, but from what I've read around here, most of the 5.0 guys say that they get better gas mileage with higher gears (3.73 & 4.10) than they do with the 3.31 and 3.55 gears since the motor just doesn't have enough torque to efficiently get the truck moving, and the 5.0 has a higher peak torque than the 351 or even the 300 I6. So I would say that the 3.55 would be better than your 3.31, but higher than that would probably be better yet.

 

1) Wouldn't that mean worse gas mileage?

2) Comparing both the 4x4 and 4x2 side by side and by sight, they look to be exactly the same save only for the rear sway bar.

3) How hard is it to rebuild a rear end using a new Ring and Pinion set?

 

Lower gears for the 302 would be a good idea (lower gears mean bigger numbers) as the 302 likes to rev higher in order to keep in the power band to keep these aerodynamic bricks moving well.


Keeping it around 2000RPM would be great, and would get you better mpg than keeping it around 1600RPM as in those RPM's the engine is lugging.

 

No some engines are just happy "reving" a bit going down the highway.

With your 302's 3" stroke unless you get above 3000rpm I don't think you would loose any mpgs, but rather you should gain some becouse the motor has more rpm to do the same work.

 

Gotcha. I suppose that would make more sense.

I guess it would also be easier to buy the rebuilt rear end rather than taking one off one and putting it on the other.

 

Swapping axles would be much easier and more than likely, cheaper.

 

any engine made to rev higher to go the same speed will use more fuel to do it.

id love to see some reports of a guy hand calculating economy with a set of 3.08's and then again with a set of 4.10's with no other changes made,using a 302 and showing an increase in fuel economy.
............this i gotta see.

 

Living in a subdivision I have to put up with an HOA. I can take apart one truck in my garage, but taking the other apart in my driveway could cost me $250 in fines. COULD. Plus the donor rear end has 200K miles on it, though it was recently serviced.

I still think my reliable bet is to go with the rebuilt rear end.

 

Ok..............

 

I think that most of us were referencing that you'd use the rebuilt unit you talked about in your first post...

If it's been recently serviced, and if it hasn't been abused in the mean time, I'd say just do the swap of axles. Unless it's been used for some serious off-road time, or if it's been used for burnouts and such, a rear end is one of those things that a little bit of maintenance goes a long way, and you shouldn't have to worry about it breaking down again anytime soon.

I'd say take the guy up on his offer (if the price is right), and just swap axles and save yourself the headache of fixing yours. Unless something is wrong with what he's offering, it should be a slight improvement over what you truck had in stock form.

 

Gotcha. I really appreciate all the advice. I just wanted to make sure I make the right choice here.

Since I'm going to be junking the 4x4 F150, would it be beneficial to take the rear sway bar off of it and put it on the 4x2? I see that it's mounted to the rear axle and the frame, so I was thinking it may be worth salvaging?

 

Load plays a BIG role in fuel usage. Especially if we're talking about the factory fuel tables here.


Only if handling matters to you.

-Rod

 

Shouldnt be too much work to install both axle and sway bar.Looks like just involves changing the link pins. You may need 2wd link pins as the 4x4 has 2"blocks between the axle and the spring

 

Actually, the frame mounts don't have bolts. I'm not sure what's holding the mounts to the frame, but they may need cut off. I can get pictures if anyone needs them to understand.