Currently I have a 1999 f250 superduty with the 5.4 triton. Trucks crossing over 77k miles. About to have a posi locking rear installed so now would be the time to change my gear ratio. I'm running 315/75/16s on the truck now with a 3.73 gears. It handles alright but I tow a lot with the truck. I can do 90mph down the highway running like 2250 rpms. It has a hard time pulling overdrive. Around 50 mph I pull overdrive at about 1300rpms. Gas mileage is a joke I get about 9. I feel like it's so low because the truck works so hard with the weight of itself and the tires. Was considering 4.10s 4.30s or 4.56? I tow a lot of weight with the truck but not to daily. I do a lot of highway driving. Let me know what y'all think about it any help is appreciated.
I have a k an n intake, throttle body spacer and a edge programmer. No cats to a flowmaster 10 series

 

 

 

Atleast 4.56's or maybe even 4.88's if you tow a trailer like that regularly,4.30's seem to be the sweet spot for stock size tires. Do you plan on staying with 35's or going bigger it smaller in the future.

 

I plan on sticking with 35s. If I went 4.56 could my gas mileage get any worse? With 4.56 what would my rpms bump up to doing around 75 on the highway?

 

I recently sold my 2003 F-250 with the 5.4 and a 6 speed manual. I was running the same size tires as you are, and I had 4.88 gears in it. Mine came stock with the 4.10 limited slip and I was going to go for 4.56 gears with the tire size upgrade to keep it the same overall, but the 4.56s were on a nationwide backorder. I had to choose between 4.30s and 4.88s and decided that I would rather have the 4.88s to maintain or maybe even improve towing performance and acceleration. I was very happy with the way the truck performed with the 4.88s, running about 2500 rpm at 70-75mph and getting 11mpg mixed driving and as much as 16 on highway trips where I kept it under 70. I would recommend either 4.56s or 4.88s, either of which should be a substantial improvement over the 3.73s that you have now.

 

Do you think the 4.88s worked so well as to increase fuel mileage because the truck by itself is so heavy.

 

Yes. The trucks are pretty heavy to begin with. Adding larger tires effectively increases that by quite a bit. Bigger tires mean the engine has to work harder all the time. Gasoline engines, with a load, are more efficient when running at higher RPMs (to a point).

The engine has a "sweet" spot for both power and fuel consumption for any given load. Whenever I tow (5 speed 5.4, 4.10 gears), it gets better fuel milage if I wind the engine up to 3500, 4000 or so when acellerating.

In the Jeep world, a lot of guys get a pretty substantial increase in fuel economy when they regear after installing larger tires; I get better mileage (even on the flat) in my lifted Jeep with bigger tires by not using overdrive. This is on the venerable old 4.0L which likes to be lugged. For fuel economy and power, you want that particular engine running ~2000-~2300 on the highway.

 

Yes, I think the 4.88s helped get the engine into a better part of the power band for driving in most conditions. As the previous poster noted, these are very heavy trucks and the engine really gets into the most powerful part of its profile around 3,000 rpm.

 

Before i decided to swap a cummins in i was goin gto go with 4.30's because i do not plow or tow much with my truck yet but doing what you do i agree and would say 4.88 gears because like said above they should come with 4.30's stock. I did yield better MPG's highway though with the 35's and the 3.73 gears mainly because after i was up to speed cruising at 60 which she would not go faster without droppin gout of OD lol i was getting i think 13-14mpg city was/is 8-10 on a good day if my math is right, i have not checked in a while she has been in the garage getting worked on for almost a year now. Stock i got around 14 average between highway and city so now my average is about 10 since i do mostly city driving now.

 

I've got an 01 F350 with a 5.4 Triton in it. I'm having the axles fully rebuilt at the end of June here, and I'm having the factory 3.73 gears replaced with 4.30 gears for hopefully better overall performance as well as for better towing power in the hills as well as for picking up to speed easier on the highways. I've just got normal factory ride height with no lift or anything and I've got factory 265/75/16 tires/rims on it. I'm really looking forward to seeing how it performs after I get it all done, especially with towing a loaded trailer on the highway. I did the calculations on the gear ratio calculator on google and the RPM range won't be much higher on the highways as it is now, which is good as I don't want to have to rev it insanely high if I don't need it to. I'll keep people on here updated on how it does after I get the gears redone.
Chevy 1500+Trailer=Probably around the 7500 lbs mark or so.

Dodge2500+Trailer=6200 lbs. Dodge had no engine or transmission in it.

Hay+Trailer= 8200 lbs

 

I used to do a lot of re-gearing of axles as a side gig and I always recommended going one step deeper on gearing than the online calculators tell you. there's a lot of energy spent changing angular momentum of a big tire that the calculators don't show, and when a stock 265/75/r16 weighs in at 50 lbs and a 315/75/R16 weighs in at 70 lbs, it's a pretty big deal to the truck's performance. short story is that if the calculators tell you that 4.10's or 4.30's will get you the same highway RPM's as your stock 3.73's, then go one step deeper down the hole to 4.56's or 4.88's for real-world same performance.

 

4.30s of 4.56 would be ideal I think. What do you have for a transmission? I have a 5.4 / 5 speed with 4.10 gears. Currently has a 210K? junkyard engine in it. I towed my 4000 pound Jeep on a 2500 pound trailer through the hills of PA and wish I had just a hair more gear. On a lot of the hills I found that 4th was a hair too tall and 3rd wasn't a problem for power but I was really winding out that 210K junkyard engine.

I did capture it on the dashcam. Steepest is around 5 minutes

 

Even though you have stock size tires you have a crew cab long bed and it seems you tow regularly. I'd go 4.56 if it was me, I went from 3.73 to 4.30 running a 285 75r16 and the truck is how it should have been from the factory. I am however installing a 4" and 35" tires within the next few weeks and right after that I'm gonna swap the 4.30 gears for 4.88's . Do it once and be done with it, don't spend money twice like I'm doing

 

These are relatively modern engines - they would have no problem singing along at 3K all day on the highway ... probably wouldn't even see a decrease in fuel economy either.