Time too time the topic of switching rear end ratio's comes up to save fuel and there for $$. Here is a little though and calculation I came up with on this topic. The conclusions will vary based on your cost and if you have 2 wheel drive vs 4 wheel drive. The calculations are based on 4x4.

If you pickup 2 mpg going from a 4.10 to a 3.73 ratio which assume you average 16 mpg and then would average 18 mpg. 16 MPG would take 62.5 gallons to go 1,000 miles 18 MPG would take 55.5 gallons per 1,000 miles. That is a savings of 7 gals per 1K miles or $21 per 1k at $3 per gallon. So if it cost $1800 to swap gears ($800-900 per axle) $1800/$21=85.7k miles or 85,700 miles to break even. Now unless you tow heavy and a lot say over 7,000 lbs I would consider going up too 3.42 ratio the cost would be about the same and if you pick up an extra 1 MPG that would mean it would take 52.6 gallons per 1K miles or 9.9 gallons less or a saving of $29.70/1k miles. $1800/$29.7=60.6k miles or 60,600 miles to break even.
So anything over 61,000 miles and your ahead of the game. The key is HOW LONG do you plan on keep your truck or really how many miles do you plan on keeping your truck??? If you keep the truck for about another 100,000 miles you would save ~ $1200 in fuel cost @ $3/gal and 3 mpg increase.

Gear changing was something More Power talked about more in the early days at The Page.
I did this calculations back when I had about 30,000 miles on my truck and did the swap at 38,000 miles and figured it at $1.35 per gallon for Diesel. It worked out that I need to keep the truck at least 60,000 miles to break even. MY goals/was to drive the truck too at least 300,000 miles. With about 110,000 miles past break even on my truck and with fuel cost well above the $1.35 I used I am way ahead.

So. I believe it is feasible to change axel ratio's out and save enough to pay for it self PROVIDED you drive the truck enough.

So if you only keep the truck for 20 or 30,000 miles after changing gears your only wasting money and helping the next owner.

A couple other thoughts about changing rear end gearing, More Power said in another thread when he did it on his old K-5 Blazer it made the truck much more pleasant too drive.I found this true also, lower engine and drive train noise. Jim in the early days talked about extending engine life by going to 3.42's from 4.10's by an estimated 100,000 miles and maybe 50-75,000 miles going from 3.73's (my old ratio) too 3.42's. At the time with a New 6.5 this was important too me. Maybe it's not as important to folks who buy a now well used 6.5TD as it was then. I do think there maybe few folks like myself who buya truck for a long time and want to keep their long term cost and headaches to a minium.

In conclusion I think changing gears is a good option for some but not all 6.5TD owners. If your considering this consider your towing profile, type of usage, and expected ownership time frame.