Many of the fuel-mileage figures I've seen posted are rediculous. I've been driving 6.2 and 6.5 powered vehicles since they came out in 82 - in all shapes and sizes - and I've been a diesel mechanic for 40 years. I have a half-dozen 6.2s on the road right now and many more off-road. I live in a mountain area of New York - but I've driven cross-country and kept good figures on fuel mileage - some in flat-land areas. A 6.2 has about the same powe as a 305 gas - and wont' do better by turning up the fuel - unless it has a turbo. 6.5 is basically the same engine with a slightly bigger bore -and turbo 6.5s have a lower compression ratio and thinner/cheaper blocks than the 6.2s. If a turbo is added to a 6.2- you'll get more power and usually use more fuel- not less. And, getting the RPMs down with oversize tires usually lowers the fuel-mileage - becasue of increased rolling resistance. Anybody who tells you they're getting 28 to 30 MPG - is either lying - or nuts.
A few figures or real-world mileage figures.
1983 K5 4WD Blazer - 6.2 diesel, 3.08 axles, T400 trans - 65 MPH - 19 MPG empty - 13.2 MPG pulling a 20' camper-trailer - both figures with standard 235/75-15" tires. With 31 X 10.50s - mileage is worse empty or pulling a trailer. Same vehicle with stock tires gets an average everday MPG of 16.
1986 K5 4WD Blazer- 6.2 diesel, 3.08 axles, 700R4 trans, standard tires - 23 MPG at 65 MPH in flatland.
1982 K10 1/2 ton 4WD longbed pickup - with 6.2 diesel, 3.08 axles, NP833 overdrive manual trans - 24 MPG at 65 MPH on a flat highway with stock tires. With 31 X 10.50 tires - drops to 22 MPG.
1987 V20 3/4 ton, 4WD Suburban - 6.2 diesel, T400 trans, 3.73 axles - absolute best of 19 MPG at 65 MPH - but 16-17 is more common. Average daily driving - 13 MPG. I also have an 89 GMC Suburban - set up just the same and gets exactly the same mileage.
1995 1 ton, 4WD pickup with dual rear wheels, 6.5 turbo, electronically controlled inj. pump, overdrive-automatic, 4.10 axles, - gets 14.5 MPG at its best at 65 MPH.
1993 3/4 ton 4WD pickup, 6.5 turbo with mechanical injection, 80 auto trans., 3.73 axles, 18 MPG at best at 65 MPH.