I have a 04 F150/XLT/SCAB/4X2 with the 5.4. It has 4.1 gears with a limited slip. I would like change the rear end gears as I mostly do highway driving, my payload is not heavy and I never tow. I think it has a 10.25" rear end.

-what's the lowest final drive I can put in it, is it 3.08?
-will I be able to keep my LSD?
-how do I re-calibrate speedo/odometer?

thanks

 

call me crazy, but i think the LOWEST you should go is 3.55, no lower.

 

Trust me, you DO NOT WANT 3.08:1 gears. Your truck will take 10 minutes to get up to highway speed. Like crash_lucky13 said, 3.55s minimum.

 

I agree. And if you do go with 3.55s, there are plenty of used rearends out there which will bolt right up to your truck. No time spent in the shop or ordering parts. You can also probably sell yours to a junkyard to get some money back, or just keep it around in case you decide you don't like the higher gears.

 

Yes, it IS doable... but it will cost a lot more in the city... but why waste the money?! Did you know that 3.08's costs wayyyy more than say 4.10's?! Why? Because there is more teeth and more manufacturing goes into them.

Yes, you will have to recaliberate your speedo.
Yes, you can keep your LSD.

I can see it for highway mileage... but nothing else.

-It limits your towing.
-It wastes gas in the city.
-Takes your truck from 0-60 mph in 10 minutes.
and more...

It's your truck, your money, and your decision... do as you please.

 

Please correct me if I'm wrong, but I'm unaware of either a 10.25" rear-end, or a factory 4.10 gearset, for the F150... Or did you or someone else regear and/or put a 10.25" rear in...

 

It's probably the #8200 GVWR optioned model with the 7 lug wheels... These "HD" F150's have been made since 1997 and started out with a #7700 GVWR. In 2004, the 'standard' F150 was up'd to #7200 GVWR and the "HD" one to #8200.

All the "HD's" have a semi float 10.25" rearend.

Lots more that go into them, but that's the basics..

Can't comment on wanting to go to 3.08's.... I want to go to 4.10's!

With 3.55's and 32" tall tires, I'm pulling around 1600 rpms in OD going 60 mph. With 4.10's I would be pulling around 1800 rpms....

Your engine likes to rev, and the lower gears will only bog your engine, making you have to give it more gas to get it moving.... I'll bet you will gain more mpg with some more conservative driving habits then by getting those gears... JMO however, so who knows??

Mitch

 

I do believe you can keep your LSD with a gear change, and the speedo change, I believe, is a different gearing for the speedo cable going to your tranny, at least that is how it usually is. If not that, a tune flasher might help with calibration for the speedo.

If fuel economy is your aim, I can safely say in this case that going from 4.10 down to 3.08 gears is NOT the way to go. You will definitely bog down your engine and make it work harder. No lower than 3.55. You got to keep your overall final drive ratio in mind when thinking gears for trannies, axles, and even be mindful of the tire sizes you buy in the future to keep your truck from burning too much fuel for hwy driving.

When you shift the rpm at hwy speeds with gear ratio/tire size changes, you are at a different point on your torque curve, and lower rpm's mean lower torque output, which would cause you to depress the gas pedal more to get the overall performance back to where you are accustomed to and that means lower mpg. What transmission model are you using? And what is it's final drive output to the drive shaft?

I would suggest efans and underdrive pulleys to take some work load off the engine to help with mpg. Would potentially be a lot cheaper than axle swapping, regardless of gear ratio. Some on here have even suggested pouring a certain volume of acetone in the tank when you fill up. I cant attest to that personally, but several others have tried it with better mpg.

You mentioned that your payload is not heavy, which implies you have a payload of some kind. Lower gears will make that payload seem heavier to the truck as it would already be working harder to pull its own weight. Any payload beyond that will cause more work required from the engine.

A buddy of mine had an old 60's model car, nothing great, it had a 3-sp auto, and 3.07 gears, I think. It also had a small standard V8. It could do over 160 mph, but it took about 10 to 12 miles of open flat road for it to reach that speed because the engine had to work so much harder than a car with taller gears.

Well, those are some of my thoughts. Let us know what you do as all of us are interested in what others do for better mpg(besides hearing that we need to buy cars instead!!).

 

ENDURA:
When i purchased my 2002 f150, it had 3.31:1 gears in it, 4.6 engine. it was great for highway driving, but thats about it. if you dont mind it taking a little longer to get up to highway speed, that would be about the lowest i would go. Your decision to go from 4.10:1 down to 3.08:1 is a big change!!
randys ring and pinion website has some calculators for gear changes, tire changes and they are pretty good in helping determine what gears and tire combo will work for you. as some of the previous replies have said, you will loose that get up and go seat of your pants feeling, and towing capacity.
you will be able to keep your limited slip, as that is the carrier for your gears
and it should be a simple program fix at the dealer for the gear change. on my truck, the computer only had a certan range for gear changes, so the dealer couldnt fix the speedo cal problem. i had to get a diablosport predator tuner to fix the cal on the speedo.
my $.02

 

city driving, I agree, it's a wash. on the highway however, I'm turning 2000rpms at 62.5mph. the 3.08 would bring me down to 1500rpm. I realize drag is drag but a 500 rpm drop in steady state cruise rpm on a 5.4L V8 has got to make a significant difference to fuel economy.

truck definitely has 10.25" rear. HD payload package.

 

It's too bad you don't have an 8.8 rearend! I'd sell you my 3.55's in a new york minute! I swapped mine out for the 4.10's because I'm sitting on top of 33's and I mostly drive city.

 

You probably won't see any mileage increase at all and anything below 80 it will feel like it lost about 100 ft/lb of torque. I am not being extreme, either when I say that. It is up to you but you should drive someone's truck with a tall gear before you do it. If you don't live on flat land you will probably wish you had the 4.10. I hated dropping speed every time the road went up the least little bit.

 

Not necessarily. There are other factors involved.

Otherwise, people who put large tires on their trucks (effectively lowering the final drive ratio) wouldn't report MPG increases when they put shorter gears in their trucks (which raise rpm).

There's a point of diminishing returns.
I don't know where that point is, but even the MFR doesn't go below 3.31, and fleet MPG is hugely important to Ford.

But there's only one way to find out...let us know how it goes.

 

You are assuming that the tranny programming would even hold overdrive in this situation. My guess is that the load seen by the drive train would cause a lot more down shifting by the tranny, which would defeat the purpose of going with shorter gears. 500 rpms lower on the torque curve would make a significant drop in output at the engine, and too much would cause the tranny to run in the lower gears more often to keep the vehicle moving at some desired speed. Our trucks put out good torque but there are limits. I had a 1997 Honda Passport V6 that I traded in to get my F150. It had 29.5" tires stock, I put on 32" tires, and that was less of a final drive ratio change than going from 4.10:1 to 3.08:1 in the axle. Road grades that I would go up in cruise control without downshifting now caused me to down shift all the time, and yes, my mpg dropped about 2mpg for hwy, more so for city driving, because the tranny saw an increase in load more often than with stock tires. Our Fords are no different, and at some point a lower gear ratio will cause even the 5.4L to bog down. However, like others have said, no way of knowing for sure until someone tries it.

With my 3.55 gears, I do about 71 mph at 2000 rpms.

 

Can you do the swap? Sure... piece of cake. $500-1000 depending on the shop and gear source. Not a terribly difficult job. The 10.25 Sterling gears might cost a bit more for the parts, but they're not impossible to find.

Reprogramming is a simple matter too. The dealer can do it permanently, or you can get an Edge, Superchip, or other programmer and do it that way. That'll bring the shift points back in line, as well as correct the speedometer. (No cables in over a decade now)

Now, that being said, I think you'd be wasting your time. As already mentioned, you'd be straying to the other side of the spectrum, where you need more power to generate the torque. I suspect that 3.08s would be so far to the other side, even on the freeway you'd see a drop in fuel economy, not to mention the detrimental affects on towing capacity and cargo capacity.

Finally, higher gearing (i.e. numerically lower) does not necessarily have more gear teeth to cut making it more expensive. Expense is not a matter of machining costs alone. Supply and demand have as much to do with the cost as anything.