Dagnabbit! I could've sworn my '99 5.4L has 4.10's in the back! My Grandpa bought it new in '98 to pull their 32' 5th wheel camper and I could've sworn he said it had 4.10's and it held highway speed without much issue pulling the camper (this with Grandpa riding shotgun, mind you ). Maybe the dealer just told him it had 4.10's to get a sale...? Either way, I got a bit curious after my buddy with a slightly newer F-250 with 3.73's and the 5.4 said he was running the same RPM as mine on the highway. I looked on the door sticker - "Axle - 31". Dang...well, maybe a misprint or something. Looked on the diff tag - "<something unintelligible anymore>...73". Well, crap....maybe he had the dealer swap it out and they didn't change the sticker or tag...? I jacked up one wheel, spun it around 2 times and the drive shaft turned 3 3/4 turns. Well, poo... It's the 2v 5.4 as well, so we're talking 260 hp and 350 ft-lbs. The heaviest thing I've pulled with it is a focus on a flatbed, so ~5,000 lbs, if that, but it held highway speed just fine up and down hills. I really doubt I'll pull much anything significantly over 7,000 lbs regularly. I figure about the heaviest things I normally pull are cars on a car trailer and ski boats.

Dropping in an Ox Locker is on my list of mods, so I figure I ought to go ahead and drop in 4.10's or 4.30's while I'm at it. My main question is how this will affect MPG's. Right now, I get just a smidge over 14 pretty consistently empty with my RPM's at about 2,100 at 75MPH (stock size tires) and it doesn't downshift very often when empty. I'm sure I could easily calculate RPM's, but I'm curious what the MPG difference would be with 4.10's or 4.30's both in the city and highway empty and loaded. I'm not terribly concerned about MPG as it's not my DD - mostly just curious.

 

I drive kinda faster in my 2004 f350 ext cab short bed with 5.4 Auto 4.30 gears and 285/75r16 tires and around town I get 11.7mpg and highway I'm not sure because I hardly drive highway

 

Well to make your day even worse your 99 has non pi heads so your talking about even less power than you thought.

 

Oh it's got port injection for sure. Ain't no way it's TBI! I looked up the specs on it for the hp/tq numbers.

 

Haha what are you talking about I didn't say anything about tbi the 99 5.4 and 6.8 had different heads than the later versions the numbers you looked up are are for f-150's and the 00 and up super duties. The heads they used were called pi heads stood for performance improved so we all call the early head non pi heads. Do your research.

 

Ahh... I thought 'pi' stood for 'port injection'. All of the research I've seen says the 5.4 from '97 was revamped in '99 to those numbers, so according to my research, it's got 260/350.

 

The regear to 4.10 would be a nice improvement. The modular motors love to rev. Your biggest friend right now are the small stock tires. You MAY see a very, very slight decrease in empty hi way mpgs, but city and towing should go up, that is if you can keep your foot out of it. It's still going to be slow, but a gear is the biggest bang for the buck towing and performance mod you can do.

 

Your right it was revamped in 99 for everything but the super duty. Your motor makes 235hp and 335 ft/lb
1999 Ford Super Duty F-250 Styleside XL Specs and Performance | Engine, MPG, Transmission
And if your going to do gears go 4.30's.