I have an '01 CTD with a rear axle ratio of 3.55 with a Dana M70 and a 48RE 4-speed transmission. I'm thinking of getting a 3rd Gen replacement. Most that I read talk about 3.73 or 4.10 axle ratios. I would like my new truck to get the same or better mileage as I get now. Is the 6-speed auto with 3.73 about the same as the 4-speed auto and 3.55? How do you compare these?

 

Still won't be quiet the same. The 6spd auto has a double OD so it's less of a jump from direct to OD like the older 4 spd autos. Installing 285/70/17's will get you really close to the 3.55 ratio (around 3.58) if using 3.73's.

MikeyB

 

I think the 6sp auto, 6th gear is a deeper OD than any of the previous transmissions. Seems like the 6.7s, 3.73 axles, I've driven are under 2krpms at 70 mph.
I believe you can get the new trucks with 3.42's as well.
Anything is better than my G56 (later one with the deeper OD too). Even with 285s, I'm over 2k rpms by the time I hit 65mph 3.73's in the truck.

 

Here are some comparisons;

48RE with 3:55's at 70mph in 4th with 235x85x16 tires 1800rpm

68RE with 3:73's at 70mph in 6th gear with 235x80x17 tires 1700rpm

68RE with 3:42's at 70mph in 6th gear with 235x80x17 tires 1550-1600rpm

The 3:42 rear is available in last years models or can be bought after market.

My truck as is runs around 20mpg on the hwy with the 48RE at 68 mph.