I have 3.73 gears now on a lifted 02 excursion with the v10 and 315/75/16 bfg tires. Motor is stock. My first question is I get about 8mpg right now, what will the 4.56's do to that? I pull a 6500lbs trailer alot. Should I just try to find a 4.30 axle and swap them or just change gears? Thanks

 

Those 315/75R16 tires come out to about 35" or so (I'm seeing 34.5" for the KO2 tires in that size), considering that and the fact that you tow I would suggest going to at least 4.56 gears if not 4.88s.
With those tires and the 3.73 gears your current effective ratio is down to a 3.41 and I'll bet that V-10 hates that especially when towing. The towing/performance optional gear ratio when these trucks were new was the 4.30, so move you up to that gear ratio neighborhood you would need a set of 4.56 (for a new effective ratio of 4.16) or 4.88 (for a new effective ratio of 4.46). I used this calculator 4Lo.com :: Tire Size Change, New Gear Ratio Calculator and our stock tire size of 31.5" to get those numbers.
I run 305/70R18s that are 35.28" tall along with a set of 4.88 gears for an effective ratio of 4.39 and the improvement in towing (and solo) performance is huge! It's like a different truck really. Our EX is the family tow rig and in stock trim with the 3.73s and 265/75R16 (31.5")tires towing our 9500lb toyhauler we were getting 6 to 7.5 MPG. After the change to the 4.88s with a set of 32" tires we got 9 MPG towing the same trailer over the same route (Philly to Savannah) at the same speeds (60-ish MPH). Now with the 35"s and those 4.88 gears we pull a 41' TT that scales at 11,000lbs at 65 to 70 MPH and are seeing repeated 8 to 9.25 MPG. The V-10 really likes working at the slightly higher RPM the deeper gears provide. Our unloaded mileage has dropped from a stock 14 highway to 13 MPG now at 65 MPH. We also run the Banks headers (balance of exhaust is still totally stock) and custom tunes from 5Star Tuning, we use the 87 Octane Performance tune for most towing with some mountain trips on the 89 Octane Performance tune.

Don't fear the gear!

 

Just answering you 1st Q (Tom took care of the rest) How do you know you are getting 8 MPG

LOM or hand calculated & neither will be correct until or unless you have adjusted the PCM for the larger tires. You can see how far off your odometer is by comparing it's reading to the mile markers on the side of the highway then EXtrapolate everything from there if'n you want to go old school.

 

I think more axle ratio will help your fuel economy in the city and may not hurt it on the hwy. These trucks are heavy and anything you can do to help it move, will help economy.

 

Thanks for the replies. Do you guys tow in OD? If so what rpm are you turning and what mph? I am pretty sure I want the 4.56 but am also toying with the 4.88's now. I would like to tow in OD at about 68mph.

 

I always use OD when towing, it's the only way to get the best MPGs and the trans was designed for that usage as long as it's not hunting in/out of OD frequently then it needs to be dropped down a gear to keep it from building too much heat.
With my 4.88s and 35" tires my EX in OD is turning 2055 RPMs @ 60 MPH, 2230 RPMs @ 65 MPH and 2400 RPM @ 70 MPH. I used a GPS for the speeds and my Ultra-Gauge for the RPM readings.
I also have banks headers and 5Star Custom Tunes which both help boost power/torque a bit and can hold OD and speeds over 60 MPH towing our 11k TT on Interstate Highway grades (in the East).
For anyone doing any serious towing with these trucks I highly recommend having some way of monitoring the transmission temperature.

Also keep in mind that nearly ALL ST trailer tires are max speed rated at 65 MPH.

 

FTE quote of the week right there.......

 

so I have pretty well decided on 4.56's. I am hoping to have it done in May. Will it need a full rebuild with new bearings and all that or can I just have the ring and pinion done? The EX has 106k on it. Also, it has limited slip now but it doesnt seem to work very well, what is the best way to cure that? Thanks

 

If your going in, He** yea put in bearings
Lock it up is the only cure

 

The factory LS can be rebuilt, but it doesn't have a very good reputation. If you are spending the money, you should consider a Tru-trac or locker.

 

Does anyone know if a carrier change is necessary for 4.56 or numerically higher? I'd assume 4.30 swaps as us but wasn't sure of anything greater.

 

I think 4.56 is as far as you can go. The 4.88's will need a new carrier.

 

Thank you!

 

So I might be changing my plans due to budget. Is there much differrence between 4.30 and 4.56? When it's all said and done I could swap axles and be in to it for about $500 for both axles or if I have 4.56 gears done I am looking at $1900. Is 4.56 worth the difference? My trailer only weighs 6500lbs. Any thoughts?

 

4.30 is fine providing the axles are in good shape, there is only an ~5% difference.