There is another recent thread covering this question- yep it does but it would take forever for it to pay for itself- I figure 4 years for what I paid for mine with gas at 2.75. Do it because it drives so much nicer esp if your auto is on the way out-
I gained no mileage with the swap when I did it. It seems that it takes X amount of fuel to move the 60 down the road no matter what. I liked that I could cruise faster (probably why I never saw an increase) and the lower first gear was a very welcomed change. I liked that more than the overdrive fith gear.
I haven't commented on my H55 since it was put in last summer. It is the best mod that I've done to my 62 hands down. After driving it with the 55 the question continues to occur; why was an auto ever put in this truck to begin with? The difference truly is night and day. Just this weekend I drove it fully loaded through the mountains on a rafting trip to So. Oregon. It doesn't lug down like it did with the auto and I'm able to crest steep grades without problem. New drivelines were installed and driving on the interstate at 70 is a breeze....very, very smooth at a little less than 3K rpm. I very seldom use the 1st gear as it is so low. Nice to have though. I agree with the above that whatever mileage increase I may have seen has been consumed by the faster speed that the 55 allows me to drive.
Just a fantastic mod and if and when your auto bites the dust this is the way to go. I also have a great mechanic in Portland that did the install. It looks like it came from the factory as a standard tranny.