I'm about to have the differentials changed on my 2004 F350 V10. I bought it new with 3:73 gearing. Now we are pulling a 11,000lb 5th wheel. What would you recommend: 4:30 or 4:56. If you have experience with these two ratios I'd like to hear about what you know. The who is doing the work says that 4:30s cost about $400 more than 4:56s. I'm mostly concerned about RPMs and gas mileage while towing. Thanks.

 

will depend on how fast you expect to go, and whether you're in flatland or mountains

 

With that amount of weight, I personally wouldn't even stop to consider 4.30. Go straight to 4.56.

In fact, if you're PRIMARILY going to be towing with your rig as you indicate, you may want to consider 4.88. Depends a lot on where you live (elevation), elevation you'll be towing at, the anticipated speeds you'll be towing (55mph 2 lanes, or 65+mph freeways), etc.

There's a guy on here with a green Ex who did 4.88 for a toy-hauler in the weight realm you have and his reported towing performance and mileage improved substantially. I can't remember the guys username right off-hand. W3EZS...something like that. Do a search for "4.88" and he's sure to turn up. Or he may chime in here shortly.