I've got a '97 Ram 2500, two wheel drive, 5.9l, auto trans, 3.54 Dana 60 rear diff, running standard 245/75R16 tires. I tow a 6600 lb. fifth wheel RV, and find that at the freeway speeds I run, the engine is right at, or below its powerband. (overdrive is, of course, locked out) 2500 rpm on the tach. shows about 64 mph. Do any of you folks tow a comparable trailer with a similar truck. From my research, it looks like there's nothing in between a 3.73 gear and a 4.10. I tow slightly below freeway speed limits on my rig, since I know that the auto tranny in this series of truck has some issues. Fortunately, mine is fine, and I want to keep it that way.Which ratio would you recommend, the 3.73 or the 4.10, and why? Thanks for any info you can offer.

 

If you tow a lot or on hilly terrain all the time = 4:10
If you tow for a month or two = 3:73
If you tow a week here and there = keep what you got.

Changing gears ratios is not a cheap job.

I have a 98 1500 quad cab 360 V8 with a 3:55 axle and tow with O/D off with a 96 Terry fifth wheel (5000 lbs.). There are times I wish I had a 3:92 axle, but I only tow for a few weeks in the summer.
So my trade off for better fuel mileage when not towing.

 

Looks like you would want to split the difference but since there's no 3.90 available ... go with 4.10's and bump your tire size up to 265's or 285's ... 245's are kinda puny.

 

With my 1999 1500 I run 4.10 rear and it works well for me. Since I am in California the speed limit while towing is 55. I tow a trailer that is pretty close to the weight you have and usually run about 60 mph. I too lock out overdrive and it runsabout 2200 rpms. I do have 265 tires though. Personally after talking to several people I chose the 4.10 since it did not seem that going from 3.55 to 3.73 or even 3.92 made enough difference. The 4.10 did make a noticeable difference on grades. Mileage while towing actually has increased slightly. Unloaded mileage varies depending on my driving but did not go down substantially. Either way it did not effect the mileage enough to make it a major factor.