Hi guys,

I have a basically stock '01 5.4 4x4 expy. I've removed the air silencer & replaced it with a tube I fabricated to fit in the fender hole. Tranny & Diffs have been switched to Synthetics. I'm thinking about making some modifications and one of those changes are the gears. My tires have been changed from the stock 16" to stock 17" tires, P265/70-17. I currently have 3.55LS gears.

1. What kind of RPMs will I see in OD going 75?(speed limit in SD!) Right now I think it's just barely over 2,000 RPM - maybe 2200.

2. What kind of RPMs will I see in 3rd gear going about 65?(I maybe pulling a ~6,000# TT soon - I figure this will be the max towing speed)

3. What kind of change in MPG will I see if I drive nice? I currently get about 12 town / 14-15 interstate. I was getting closer to 16-17 with my stock sized 16" tires on the highway and steel rims. The 17" tires feel much lighter with the aluminum rims even though they are bigger. I'm kinda wondering if the bigger tires with the 3.55s are a bad combination? The tires are the stock Goodyear Wranglers and they were used.

4. What kind of re-programming will I need to do? Can I do all of this with something like a predator? (It's gear-ratio & shift points that need changing right?)

Where can I get the gears and how much should I expect to pay? I think the ones for the front axle are going to be ~$200 (8.8) but the rear looks to be $500?!?! (9")..also, the 8.8 is stated as 4.10(8.8) and the rear is 4.11(9") - do these work together?

I'm looking at e-fans, underdrive-pullies, shorty headders/catback and perhaps a new intake tube as well. I wish I could do it all but I don't want to spend that much money! The gears may end up being a little more than I want to spend but have the impression if I end up towing a big TT this would be the best upgrade.

Thanks in advance!

 

I'll give you my experience with an 03 5.4L/3.73 265/70/17 combo (& in SD).



1. I turn around 2000 rpms too.

2. With overdrive off, it turns 2600 rpm at 65 mph (sweet spot for 5.4L)

3. I get 12-13 puttering around Sioux Falls. I get 16 on the highway without wind. 8-9 mpg towing 4500 lb travel trailer.

4. Don't know about that. From what I read on towing forums is you pay for the gear swap and a couple hundred to reprogram.

I don't know where to get the gear swap done if you plan to do it. Usually no more than $500 per axle. When I needed a rebuild for my Jeep, Inland Truck had the best deal for a Dana rebuild.


If you are going to seriously tow, gears are your best investment. Torque & horsepower is wasted if you can't get it to move the truck.

 

If your rear axle is stock, it isn't a 9". Some guys have talked about $1200 to have gear changes done. Probably a dealer or something.

Go here to fiddle with different ratios, tires etc.

Scroll down for rpm or mph.

http://www.novak-adapt.com/knowledge/gearing.htm

 

Thanks for the replies guys. The RPMs don't sound too bad Eric. I guess my biggest fear is that I get these gears changed and end up seeing a 2-3mpg drop in daily driving. I don't see changing from 3.55s to 3.73s worth the cost of labor involved. Maybe my thinking is wrong? I'll have to do some checking on the labor, I may know someone that can do it for me.

I like the calculators you found 85e150, fun to play with. After sitting down to Google I found another neat gear ratio calculator: http://www.jeeptech.com/javaAps/gta.html

The 4r100's gears are as follows:
1st = 2.71
2nd = 1.54
3rd = 1
4th = .71
Rev = 2.18
Transfer case: High = 1, Low = 2.64

It generates a graph of engine RPM / MPH for all gears and a table that shows MPH based on the RPM you enter.

My rear axel is stock. I'll have to look at the shape of it & verify its the 8.8. I was thinking I read somewhere it could be the bigger 9.75".

After looking at Fordracingparts.com for a while I found the following page: http://www.fordracingparts.com/parts...tKeyField=3631 - I'm thinking if I do this I'll go with the Ford gears. I find it odd they don't have a 4x4expy selection under the trucks section yet once I found the gears they have a note by the 8", 9", 10" gears section - "These ring and pinions have been developed to increase towing capability of 5.4L Expeditions" *shrugs*

 

If you don't know what you're doing, you'll want to have the gears installed by a pro. I recommend calling around to some of the local off-road shops to get prices. Be sure to add a "good" limited slip diff to the deal as I'm sure your factory one is about worn out and they aren't that great to start with. It's cheaper to do it now than to go back into rear-end later on. You'll also want to put in new bearings & seals... again, much cheaper to do it while the gears are out vs doing it all again later.

Don't be too shocked by the price quotes you get. Changing the front diff is pretty labor-intensive... thus expensive. I paid about $1200 for bearings/seals, 4.10s & an Auburn LS installed back in 1998 for the '97 Expy we owned at the time.

On edit:
Your front is an 8.8" and your rear is a 9.75"

 

You can estimate your RPM change at any given speed by simply multiplying your current RPM by 1.15, as 4.10 gears are about 15% shorter than your 3.55s.

(There are online calculators that will do this, but most require way more info than is required and are overly complex. You are only changing one parameter - final drive - so everything else is the same on both sides of the equation and thus cancel-out. So you don't need to know tranny ratios, tire sizes, or anything like that unless you are changing them)

The change from 3.55 to 3.73 is only about 5%, so this is not worth it.

I run 4.10s on street-sized 285-60R18 tires, and it's perfect. both for towing (7,000-lb boat/trailer) and for just driving - lots of power, no tranny gear-hunting on grades

BTW you should have a 9.75-inch rear. Ford didn't make a 4.10 back when I had mine installed (not sure if they do now) so I got an aftermarket one, Yukon I believe. Definitely put a LSD in there while you've got it apart.