i have a 2003 f350 v10 with stock 3.73. I am thinking of swapping to 4.30 to make the towing experience a little nicer. being as how its my daily driver i am wondering if anyone has any experience with the fuel mileage difference between the 2 loaded and unloaded. I average about 10.5 mpg combined now.

 

I have the same truck and would like to know what RPM's it would be at 70 mph With the 4.30's ?

 

I have the 4:30's. At 70 its about 2200 rpm's if I recall.

MPG should not be affected much. I average right at 11 unloaded and towing lightly, 4500 lb boat, I still get 11.

The gear change will completely change your truck and be the best bang for the buck you can do.

 

See notes above

 

What are you towing?
We tow a 41' 11,000lb TT with our 2V V-10 4R100 4X4 EXcursion.
Back when it was still factory stock with 31.5" tires and 3.73 gears we towed a 9,500lb toyhauler and saw 6 to 7 MPG loaded to about 17,500lbs CGVW, unloaded on pure highway trips it would get 15+ MPG and with my typical 50/50 mix of city/hwy commuting it got 14 MPG.
I then swapped in 4.88 gears and bumped up to 32" tires for a couple of towing seasons. Over the same routes (Philly to/from SC) and at the same speeds (60/65 MPH) pulling the same TH we got 9 MPG. Unloaded the highway MPG dropped to 12 and the mixed was down to 11.5 MPG, but it pulled like a locomotive and felt like a hotrod unloaded!
Currently I run 35.28" tires with a 4" lift, which gives me an effective 4.39 ratio. Unloaded on pure highway at 65 MPH it now gets 12.5 MPG, some of that loss is due to the bigger tires and lift as well as the gears though. It doesn't see any commuting miles these days as it now serves as our dedicated tow rig for the new 11k TT. We typically now see between 8 and 9 MPG on the longer towing trips with a best of 9.4 MPG over 420 miles. It handles the big TT very well as we tow all over the Eastern states and it can hold OD on all but the biggest Interstate grades.
The EX also runs Banks headers and custom tunes from 5Star tuning (I run the 87 and 89 Octane Performance tunes while towing), both of which made positive improvements to both general and towing performance. But the single largest performance increase was from the deeper gears.
With only a gear change from 3.73 to 4.30 I would expect to see no more than a 1 MPG drop in unloaded mileage and a similar sized increase in towing MPGs along with much more time spent in OD while towing and better hill climbing ability and acceleration.
On my rig I verified my RPMs with the digital tach on my Ultra-Gauge and my speed with a GPS with both the 32" and 35" tires, so both an effective 4.80 and 4.39 ratio. All readings were in OD and TC locked up.


Speed.......4.80.........4.39
50............1930.........1720
55............2075.........1900
60............2280.........2055
65............2430.........2230
70............2600.........2400

 

What can one expect cost wise in a 3.73 to 4.3 upgrade in a 4x2, no diy (beyond my paygrade)?

 

So ... if you want to know what RPM you're going to be at with a different gearing (and the SAME SIZE TIRES), do this:

Go out on the highway, see what RPMs you are running at now at any given speed. Write it down. Let's say, you see 2200RPM at 70MPH and you're running 3.73s

Take the RPM, divide by the CURRENT gearing, then multiply the result with the NEW gearing.

So if you're running at 2200RPM at 70 with 3.73's, you'll see 2536 with 4.30's.

Im50fast, stock van tires are smaller diameter than the Superduty. So you'll see higher RPMs for any given speed, with the same gearing. Also, remember the Torqshift might have a different overdrive ratio than the 4R100, so that'll also affect the RPMs.

 

Is it a 4x4?

 

Unloaded on the freeway I will drive 75 to 80 and get 11 mpg.

Towing the boat, regardless of the road, I will keep it to 65. I still get 11 mpg.

No offense taken. I should've clarified that.

Another thing to clarify, I am running 285/70/17 tires and had the dealer reflash the computer. Speedometer is pretty much dead on vs. GPS.

And it may be closer to 2250 at 70 mph, but no more for sure.

 

That depends on what your local reputable shops in the area charge. Make sure you go somewhere that has a good rep and regularly does axle rebuilds. One good shop local to me quoted me ~$1,500 for a 4x4, so roughly half of that for 2WD.

 

So I'm looking at the parts breakdown and the only difference I see in parts is the ring & pinion and the drive shaft. Surely that can't be it?

DIFFERENTIAL for 2000 Ford Excursion

 

Not sure exactly what you are questioning here, but yes, the ring and pinion are what makes up the gear ratio we are talking about. A shop will typically do a full rebuild of the differential with all new bearings and seals when they change gears for you. Around here a good shop wants about a grand for a single axle regear.

 

Ah, thanks!

 

You will love the difference in gears! On my 01 F350 with a V-10....I changed from 3:73 to 4:10's.......wow! Tow a 36' 5th wheel at about 13.5K.....truck is 6800....and get right around 7-9 towing and about 11-2 empty.

 

I changed out the rear end gears in my first (1999) V-10 truck. I recall it ran around $1000 with new bearings, seals, gear set and labor. It made a huge difference in towing my fifth wheel trailer, and had very little impact on mileage. As I recall my towing mileage actually got a little better as I was shifting up and down much less. My unloaded mileage may have gone down a 1/2 mpg or so, but I never looked back. When I ordered my 2010 F-350 V-10 I got it with 4.30 gears from the factory. The V-10 just tows so much better with the higher ratio.