Last year I purchased a 2003 7.3L CC with 145k miles. The truck has a 6" lift, 315/70 r17 tires and super chip tuner. After purchasing it I quickly came to the belief it has 4.56 gears in the axles.

I use this truck to haul my jeep (<8k lbs loaded) and take it unloaded on road trips (about a 50/50 split). In the past year I have only put 5k miles on it but that may reach 10k some years). I do not plan to use this as an offroad toy.

Towing the jeep I get 12 mpg around 65mph , and unloaded I am getting around 14 at 65-70mph.

My approximate speed/RPM are as follows:
60 MPH / 2000 RPM
70 MPH / 2250 RPM
75 MPH / 2500 RPM

To better suit this truck to my needs I am considering regearing or swapping axles with different gears. I can do all the work myself but would prefer to do it once. Additionally I am considering reducing the lift height to a 4" or a 2"/leveling kit. I do not plan to decrease tire size.

Questions:
1) What RPM range should I be targeting for...
A) Best MPGs unloaded at 75MPH?
B) Best Towing performance for 8k lbs on the east coast (flat and small mountains 65-70mph)?

2) What gear ratio would you recommend for my intended use?
A) 3.73s
B) 4.10s
or....?

3) How concerned should I be running unloaded at 2500 rpm for 3-6 hour trips? I have read some about EGTs but do not have any additional gauges to monitor boost, EGT, or trans temp.

Please note my primary concern is cruising RPMs vs towing performance. Increasing MPGs is a plus but not a significant factor due to the small number of miles I put on the truck.

Thanks in advance,

 

my 3.73 gears at 65 mph turns my engine at 2000 rpm tire size in my signature.

 

2500 RPM is NOTHING to these engines from a reliability standpoint and you'll find that towing performance at that RPM is actually better than what you'd experience running at an RPM closer to (or lower than) 2000 RPM.

I'm not touching ANY economy concerns.....

 

I run 315's with a 2 inch leveling kit. I have 3.73's and tow 9-10lbs several times a year. Right at 1950-2000 at 70 mph. It tows fine.

 

Thanks for the input. I wont lie, a few extra MPGs would be nice, not to mention having a pair of 1 tons for a new project...
I think Ill keep my eyes out for a set of axles with 4.10s but until then wont worry about it.

Were 4.10s ever offered from the factory in the SRW axles with disk brakes?

 

Stock gearing, stock-ish tire size, 3.73 4X4:
55 MPH @ 1600 RPM
65 MPH @ 1900 RPM
75 MPH @ 2200 RPM

16-19 MPG Hwy (empty), depending on my mood (driving style), season, tire inflation, and road conditions.

 

I'd like to have 4.10's rather than the 3.73's....I just believe my OBS, with 4.10 and less power did just as good, and driveability at 65mph was better.

 

So you said you believe it has 4.56's. The first thing you need to know is what gears are in there for sure. An easy trick is to spin rear tires while there off the ground of course and the tranny in neutral and for 1 tire rotation if the the axle spins 4.5 times it probly got 4.56's. If it spins closer to 4 turns it probly has 4.10's, you get the idea.

And my opinion is that if you have 4.56's the jump down to 4.10's might not be big enough to warrant the money/effort, but I understand the concern with going too far and losing towing performance.

I have 4.10's and 32" tires. Running empty at 2500 rpm my egt's are around 600 so that's not a problem. Empty I get 15-16mpg 70mph and 14-15 in town. Towing 13,000 pounds at 65mph I get around 8mpg.

 

If you could find a guy like me who would absolutely be glad to swap you gears it might be worth the time. I have 3:73s running 38s and have been wanting to go to 4:56s for awhile now just haven't had the funds. I would trade you gears in a heartbeat.

With my foot to the floor my truck doesn't shift to OD until I hit 90....


EDIT: no one in there right mind that has a diesel back round or knowledge of the 7.3 would put a set of 4:56s on a truck with 315s/34.5" tires.

 

I run 3.73 with 37's so my RPM's stay relatively low. Good thing everywhere I need to go from my house is all highway. I lose a little mileage in town but make up for it on the interstate. His is a crappy photo of my RPM at 65mph.

 

I say you have the right gear for towing heavy. It all depends what you want more. Good towing, or around town and highway driving unloaded. Those MPG aren't that bad. And its a lot better on the trans with heavy loads with those gears.

 

If your not planning on towing real heavy I would go with the 3.73 gears. These should get you better mileage as you know & you can still tow up to 11-12k .Dont know about the lift as far as mileage as I am stock ride heightexcept for a 2" front leveling kit & 2 wheel drive

 

That's your opinion. It all depends on what the owner does with the vehicle.

 

To me, that's useless.....

But if it makes you happy, roll with it!