i would like some first hand real world mpg's from your rigs, also if you don't mind tell me alil about your set up like 2wd/4wd, how difficult was your swap, tranny, mods, rear end ratio, type of truck.
I can't give entirely accurate because of various, but my mileage is ridiculous good. I have a full sized '67 Dodge 2WD, Cummins 4BT [not aftercooled] stock, running thru NV4500, to 4.1 Dana 60 HD, I think about 30" tires. Pulling another truck, Chevy LUV, both trucks loaded with tools, scrap metal, parts etc. it came out to about 19-20 mpg over 700 mile run. I'd guess that load at 11K-12K lbs, maybe more: 7/8 freeway, 1/8 wild mountains roads.
I live in very mountainous area and running to town empty, truck as described above, weight about 2.6 tons for truck, I see very high 20's with the pedal to the metal. I have no doubt that 30-32 is easily possible on flat freeway runs with a light load, no fat chicks. Easily cruise at 75, but much prefer 65, broke speedo at 87-90.
Nothing seems able to overheat it. Stock has surprising power in my setup but will be tweaking soon [I keep telling myself]. Approximately the same power as a strong Dodge 360, a little more in some ways: torque!
26 mpg regular driving, hwy/city combined. Havnt had a long enough trip for hwy only,800+ miles between fuelups. More power than the good running 400 that I pulled out. With trailer even more difference. Running 3.55 but with the weight of my truck I strongly believe 3.73 would be ideal and increase mpg/power even more.
10L/100km. i have a 2wd '77 F100. it weighs 2 metric tonnes. with 3.25 diff gears, and 31" tyres, driving empty at 90. plans are afoot to swap to 2.75 gears. will sit on 100-odd easy then. pulled 3.5 metric tonnes fairly easy the other day. when working, it gives about 12L/100 k's.
I'm still running the P30 around, so this is probably a "worst case" (or close to it) for fuel mileage..
Vehicle is an 82 Grumman P30 with a 15' box. Engine is CPL 857 (120HP, JWAC) with TH400, 4.10 diff and 8R19.5 tires. Took it across the scales when I filled up last, with a full tank (and about 100 LB of misc stuff inside, but I was NOT in it..) it came to just under 7000LB.
Took a ride recently that was 260+ miles round trip, 180 miles of it freeway, the rest city / back roads. NOT flat ground running, either. On freeway I ran it at the governor (3200 RPM approx) to have 75 MPH.
Anyway, end result was right at 12MPG. Considering it has the frontal area of a bus, and this is running WAAAAY out of the ideal RPM range for this engine for economy, that's not bad. I've had similar size / weight gasoline powered RVs in the past that were good to get half that.
Performance is actually quite good in the lower to mid RPM range, but does taper off at higher RPMs. Since it's had nothing done to it other than adjusting the high idle screw for more RPM, that's to be expected.