Tire size/RPM/ Economy?

[aerialimage]

looking for knowledge from those who know.
I have a 99 3500 DRW QC 4x4 auto 47RE and 4:10 axle ratios and it is time for tires.

I always wanted to get the rpm down 2-3 hundred for economy.
I have to run at 60 mph now for about 2,000 rpm and about 20 mpg. The overhead says 21 or higher but it delivers a real 20 mpg. of coarse the rest of the world is traveling at 70. I can bump it up to 65 and 2200 rpm but the mpg falls quickly above 2,000 rpm.
my truck is stock with 4:10 ratios and 215/85/16 tires. and I have drum rear brakes.
Tires won't be cheap so I need to live with what I choose.
I see most of these trucks are running 235 tires? Is this my answer? I am real happy with my mpg at 60 and even up to around 62 or 63 but anything higher than that and I am dropping economy pretty quickly. I do not need to go 75 but 70 and economy would be great.
I tow a fifth wheel, Plow snow and tow some other trailers all under 10K
Opinions please?
Also happy to listen to any tire recommendations. Currently running dry rotted Michelin ltx tires.
Thanks for any advice.
Also I have owned 6-7 of these ram diesels all from 1999 - 2001 and none of the other ones have ever delivered this economy. that is why I still own this one and the others have come and gone.
I don't want to mess up a good thing but Can I make it better? these are really small tires for such a capable truck!
thanks for any help. really need to buy tires in next week or so.

 

[JR]

I can not help you much with dually tires as I am not familiar with what will work and fit in your application. But I have had SRW trucks, both with 4.10 And 3.55 Gears, I currently have 3.55s and I would rather have the 4.10s. Running stock sized tires the 3.55s are OK but I need a wider and more aggressive tire for my uses which ultimately gives me too high of a gear ratio especially for towing. I would really like to buy a 3.73 Gear set but it is hard to justify the cost for the little bit fuel savings I would get back.
Back to your question, it would probably benefit you to bump up one tire size, maybe two at the most. But as you add rotational mass you also decrease economy. You may drop rpm a little but you will also be increasing engine load, which will be magnified the heavier you tow. It is a bit of a balancing act for sure.

 

[Wertles]

When I went to 19.5's I lowered my rpm's a hundred or two at cruise and picked up 1 mpg. I added a smarty jr and picked up another 2 to 3. This was on my 06 with 4.10's. I don't buy into the heavier tire decreasing economy on a 4.10 truck. My 19.5's were 135 pounds a piece and I gained, not lost mpg. Rpm's were killing the fuel economy at cruise, not rotational mass. The 19.5 tires effectively lowered my 4.10 rear end to a 3.89 according to Rickson.

 

[JR]

I think 19.5s are a different comparison in this situation. The tire compound is so much harder then on an E rated tire that the decreased rolling resistance has to offset the difference in additional weight at least a little. Any time you compare a taller wider e rated tire to a shorter narrower e rated tire this will not be the case.
In my former truck with 4.10s I went from a 265/75 to a 285/75. The empty fuel mileage was fairly close depending on my driving conditions and speed but I definitely lost mileage anytime I was hooked to my trailer.

 

[NIsaacs]

The difference in fuel milage will be significant going from 60 to 70 mph. Larger tire size and lower rpm will not help much in this case.

Nick