mijotu

Hey Folks,

I'm in need of a rear end. Given my budget and timeline, a good salvage unit should fit the bill.

I have a 2004 5.4L Lariat RWD with an H9 (9.75" 3.55) -and- am about to purchase a 7200lb Airstream for occasional use (5-10K miles per year). Since I have no experience towing heavy loads with this vehicle, I'm not in a position to complain about its current attributes.

Question is: Do I take this opportunity to go to a "better" rear end? What are some of the drawbacks and advantages?

Truck is mostly used around town, weekly highway use, said occasional towing. Stock tire size.

TIA!

srb1194

I'd personally recommend a 3.73 ratio. I routinely tow 3-4 thousand pounds. I think this ratio would be best. I can get decent mileage if I drive near the speed limit.

mijotu

Thanks, srb1194!


Forgive me for asking, but maybe you can tell me: How does 3.73 compare to mine (3.55) as far as drivability, mileage, etc., both with trailer and without? What am I gaining, and what am I giving up? I don't doubt you at all — I simply have no experience with gear ratios in F150s, and you've lived it!


Thanks again,
mijotu

srb1194

http://www.ford-trucks.com/forums/sh...8#post16441748

The link above is from another forum. There's some relevant information there to review. As the ratio number increases so do your rpms at a given speed. I'm around 1950 at 70 mph. If I had a 3.55 I'd probably be around 1800 rpm. I know it doesn't sound like much but it does effect your highway mpg as well as city. I would think your preference would be to gear for the towing however others may suggest the 3.55 and just turn overdrive off when towing. You'd probably have to do that either way but everyone looks at things differently. I don't like constant shifting on hills and therefore if I'm towing I usually just turn overdrive off. Of course there are times I'm turning upwards of 3000 rpm but that doesn't bother me. It wouldn't hurt to reference a towing chart for your vehicle with that gear to see what it's rated for. My truck also has the transmission cooler that came with the max tow package. I hope this is helpful to you.

PerryB

Going from a 3.55 to a 3.73 is a 5% change, and you'll never feel the difference in terms of power/acceleration. If you are going to spend the money for a performance upgrade, get a 4.10 gearset. You might experience a small reduction in fuel economy, but the truck will be much more lively and responsive when you tip into the throttle. I have 34" tires on mine and went with 4.56's (originally had 3.73). It cost me about 1.5 mpg but the throttle response is outstanding. These trucks have a 4th gear ratio of .7 which equates to a 42% overdrive. Don't fear the gear. Thats a significant chunk of trailer you're about to hitch onto.
To put it into perspective, the 3.55 vs 3.73 is mathematically equal to the difference between a quart and a liter. Nearly nothing.

mijotu

I've hung onto every word. I can only hope my efforts in such forums are as well received as yours.


Speaking of RPMs, my background is in wrenching on—and enjoying—European sports cars and motorcycles...I'm hardly averse to the revs! I saw some recommended ratios well into the 4's, so yours seems fairly conservative compared to those—I look forward to reviewing your link to further ferret those options out.


I, too, am fortunate to have the HD towing package. We chose the Lariat mainly for creature comforts, and now it is serendipitously just the right rig for our recreational aspirations.


For your input, I once again tip my hat.
–Michael

mijotu

Hey Perry B! You snuck in there while I was waxing poetic.


This goes further to reason considering a gear change — I can only hope as many options are available locally as have been suggested here.


I think I could sacrifice a bit of mileage for the sake of performance. It's encouraging to know that the somewhat lackluster encounters with my throttle while towing in the past might very well be remedied by math.


I appreciate you sharing your experience in how it relates to mine, especially given how satisfied you seem with it!


–mijotu

PerryB

You will HAVE TO sacrifice economy for power
It's physically unavoidable.

mijotu

After doing some poking around, looks like 3.73 is as large as I can go while keeping it to 6 lugs. (4.10 goes to 7-bolt pattern.)

My final question: Will changing diffential ratio upset my speedo? If so, what would I need to do to correct it?

casey.mikl

You can do a 4.10 geadset in the current diff housing. To correct the speedo you will need a tuner to adjust for axle and tire size