Our 1999 3/4 ton would get as good as 23 on the highway. It was a whole lot less truck than the new ones and you could hardly breath the fumes if you left it running in the garage for more than about 30 seconds.
2011 F350 SRW short box 4x4 CC 6.7 PS King Ranch
B&W TurnoverBall and Companion
2003 Citation Supreme 34 RLTS
My 06 2wd dually with 4.10s gives me 19 highway, 15 overall, and about 11 towing. Overhead reading is almost exactly 2 mpg optimistic.
X2. About the typical numbers I see with my '05 3500 SRW with automatic and 4.10 rear end. For extremes: As bad as 9.5 towing the trailer and as good as 21 mpg @ 65 mph on the interstate empty.........and the DIC in mine is also full of it by about 2mpg to the high side on a consistent basis.
Over a one-year period of a 700-mile roundtrip commute between two Texas cities (and traversing Houston on each leg) I had an average that, at 58-mph/1,725-rpm that never fell below 24-mpg. Around 10,000-miles. Highs of 27-mpg on a dead stock truck. Loaded, empty, day, night, traffic, fine weather or bad. And combinations of all of those. I found it consistent with other Southern owners of 2WD '03-'04 CTD's, private and commercial (truck spec, climate, terrain, and use; the main factors for comparison as outlined by Cummins and Kenworth). It is easy to trip plan, now, with that number as a "high" for solo highway travel.
I find that mpg loses a fair amount of meaning compared to using cpm (cents-per-mile) calculations to understand operating costs as it is then fully compatible with purchase price, depreciation, finance charge, tax, registration, insurance, etc, calculations for a family budget. Same for all the line items of vacation expense.
For example, in looking at nationwide ads for travel trailers I can use 20-cpm for fuel costs (solo and towing median) with $4/gl diesel for roundtrip cost estimates to go and buy a good candidate. Or, simply, that solo highway is 15-cpm and towing 25-cpm. It's fun to be able to claim that the average the past 37k-miles is 22-mpg, but a bit more sobering to see the total gallons used and at what cost.
While a CTD is great (the best of the TD trucks the past twenty years), longevity and reliability mean just a bit more than lowest fuel burn. That they coincide (in some model years) is icing on the cake.