I've got a 7500 pound gross 5th wheel trailer that I occasionally tow, I have stock 3.55's and 265-70r17's with a Hemi and am considering either a full audio system (HU, amp and 4 speakers + sub) or a set of 4.56's in the front and back. The cost will be about equal, as I can do my own audio, but not my own diffs.
How much did your highway mileage change when you went from 3.55
s to 4.56's?

 

I have an 06 Power Wagon with 4.56 gears and the mileage sucks. It is a 3/4 ton so you might do better. My 99 2500 with the 360 and 4.10's gets way better mileage. I couldn't believe it. I still love it though.lol.

 

thanks - if I don't get the firewall stuck to the bottom of the right-hand pedal, I''ll get 16 - 18 on the O/H trip meter at 100 - 105 kmh without a headwind (I know that's in US MPG), Towing the trailer at the same speeds, it's down to between 8 mpg's and 6 mpgs.)
I suspect my highway RPM's at 100 k with the 4.56's would be around 2300 in OD (just like my 3.55's with OD dis-engaged).

 

In changing from stock 3.92 gears to 4.56 gears after installing a 4" lift and 35" tires, my in-town fuel economy went up about 2 mpg and my highway mileage went down about 1 mpg. Lot better performance with the re-gear, well worth losing a lousy mile per gallon over the interstates. Also the tranny doesn't shift "searching" for the right gear hardly at all anymore when traveling...