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Just a speculation but I would guess that hilly country has less wind than does prairie landscape. Biofuels are also more likely to show up near typically windy flatlands because the crops are raised there, so Biofuel use may be likely less available in the hills. So fuel economy may actually seem, or even be, worse on flatter roads since hills may be a wash and not be as bad as wind.
Furthermore, if the hills are not too steep, mileage might even improve for another reason. A diesel engine gets more efficient as the turbo loads effective compression ratio upward. On the mandatory corresponding downhill, the engine idles (very thrifty in diesels) as the rig coasts back to the bottom. Where this extra efficiency would not be the case is when the hills are so steep that braking (engine or brake shoes) must be engaged to maintain safe speeds. In that case part of the saved "fuel-heat" energy during driving uphill is wasted by the useless heat dissipation by the braking system going back down.
Running a diesel engine at high load specifically for efficiency is demonstrated by the fact that we were often required to disable some multiple locomotives in our rail consist so that all the required HP was produced by the least number of engines, therefore the fewer running at higher load levels. According to railroad engineering studies, it saved a significant amount of fuel. So it seems quite possible that cylinder cut-out on our single engine diesels would pay off, and I surmise, is the same reasoning gas engines have been doing so (at lower power levels) for a few years.
Some of the earlier V8 Ford diesels (6.9L, 7.3L) did not have turbo's, so the static compression ratio was set higher than later turbo'd units. Intermediate engines also did not have intercoolers to "waste heat energy" to the atmosphere. I assume the same is true of the early Cummins designs. Both series of engines, seldom needing to run at max power, seemed to get the maximum fuel economy possible under ordinary light towing conditions, and especially empty. They were gutless wonders though, and consumer demand for more available power soon thwarted fuel economy as usual.
My F-250 only gets about 11-11.5 towing in low wind (not often in ND). And the tank is a PITA to fill (Got shut-off backwash 1st time on my shirt until I learned). I once drove an earlier F-250/slide-in camper with the 6.9L at 55 mph/no-wind and got 15 mpg. By contrast, my old 1/2 ton Chevy 400 gasser got 14 empty on a good day,
maybe 9 with an 8' slide-in.
Wes
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Days spent camping are not subtracted from one's total.
- 2019 Leprechaun 311FS Class C
- Linda, Wes and Quincy the Standard Brown Poodle
