Diesel mileage in the older diesel engines like my 7.3 (444 cu. inch) is very good Al, significantly better than with a gas engine. My truck weighs about 8,000 lbs empty and still gets 20-21 mpg all the time, even with 234,000 miles on the clock. I have a 6 speed manual trans in it too which helps with the mileage. The 5.4 gas and V-10 gas which were the other options for my truck when I bought it, get roughly 14 mpg and 11 mpg respectively. With my 38 gallon standard fuel tank, I can run my truck 760 miles to empty. A 5.4 gasser on the other hand can run 532 miles. From a road trip prospective, I can drive 4 more hours and over 200 miles further than the gasser. I go to my buddy's hunting camp in North Rome Pa, 205 miles from my driveway and come back again and still have about 18 gallons of fuel left in the tank.
Diesel engines are generally more efficient and run a very long time because they produce their peak torque at very low rpms which is why they are used in large trucks and equipment. My peak torque is at 1650 rpms, only 1,000 rpms over idle. My motor was nicknamed the Million Mile Motor and it will probably get there. My truck body won't be there with it though.
Heres an account of a 1.3 million mile 7.3 with my same 6 speed transmission.
The big news is that the pickup has its original 7.3-liter V8 diesel engine mated to the factory manual transmission.
www.motor1.com