I just finished a 4,000 mi trip to the 4-Corners area visiting Indian ruins. The engine is a 261. Typical road speed is 60-65 and down to maybe 35-38 on a 6% upgrade, about as fast as a typical PNW log truck. We're carrying a 11 ft. Avion Camper and last Cat Scale ticket showed 9350 lbs. It averages 10 MPG. Eleven between SLC and the Oregon line with a tail wind; 8.7 across the Blue Mtns, of Oregon.
In overdrive at 60mph I'm turning 2500RPM and 2-5 in. Vacuum depending on altitude. I put a pyrometer on it this trip and the Exh. temp runs 1200-1350. 265/75x16 rear tires and 4.57:1 axle with the brownie 1.24:1 under 1:1 direct and 0.87 overdrive. I don't expect I'll ever see mileage in the teens but you never know what more twiddling with carbs might bring.
Years ago I changed the power valve spring so it starts to open at about 5 in. HG instead of 12 in, figuring I'd get better mileage. Now, with an exhaust temperature gauge I think that's wrong and I should go back to stock settings because the exhaust temperature begins to rise alarmingly once the power valve shuts at 5 in. Hg. I don't have an A/F gauge and I can see the utility of that but the Pyrometer came to me free so I put it on.
The previous engine, a 235 ran about 200,000 miles before it burnt a piston when the thermostat stuck near the Salton Sea (minus 200ft. elevation). Same load, carburation & cam, a bit lower compression, 8.2 rather than 9:1 That it ran that long tells me my tune isn't too far off but there's always room for improvement.
