I'm sorta surprised no one mentioned what rear end ratio they had installed until the last post here from Jimbo. Gear ratios (or really the whole trans, axle and tire size combo and resultant hwy speed & RPM) is sort of a key item here.
The original post from Irish_Alley doesn't surprise me considering stock manifolds and emissions choking the engine. His follow up about the engine originally getting 13 mpg in the 86 he pulled it out of makes me wonder if there was something noticeably different with trans, axle, or tire size. The weight difference will have some effect, but not that much. If the pic from his signature is any indication, a lifted 4x4 with big tires is going to make a noticeable difference compared to a 86 1/2t 4x2. Turning 3/4t or 1t running gear compared to a 1/2t will make a little difference too.
Captkaos, that's some pretty good mileage on that R-10 305/700R4. What tire size and rear end gear do you have? Any canopy or tonneau cover? Lifted/lowered? Stock or mods?
To add to the other data points out there, my stock 87 R-10 stock TBI305/700R4 with 2.73's and 225/75R15, and a 32" tall shell (about 11" above cab) near sea level on Seattle's typical 87 octane brew with unknown % ethanol (guessing 5-10%) gets 16.5-17 mpg hwy.
Considering how tall my camper canopy is, been toying with the idea of experimenting with VGs (vortex generators) if I can find a good quick way to measure change in power required. I've thought perhaps manifold pressure, but If there was a good scanner to plug into the ALDL and display live injector pulse width or similar fuel flow parameter that could work too. (eventually will do some basic engine mods for mileage, but I'm an aero guy so...)
I agree with SUX2BU99, today's trucks really suck for mileage compared to where they could be. The trucks are porkers weight wise, and only recently have they approached Captkaos' 22 mpg. There really hasn't been much effort to extract more energy out of the fuel burnt. About 70% of the fuel energy goes out the tailpipe as heat. We haven't even tried to apply 1940's technology to extract more energy out of the exhaust.