You’ve got time in the saddle with big diesels, mine is small by comparison but some things are similar.
Here we divert into philosophy: The Truck Is The Job. The truck is in charge. The driver is
only the eyes the truck lacks. The driver is the employee, the indentured servant. Period. Sounds simple but isn't as experience counts. It's a wholly different approach. One might appreciate that a company driver is pretty good at making all his transmission shifts and can get across the continent without incident (not easy at first). The difference between him and an experienced O/O is like this: on a steep grade one knows one will downshift. A heavy truck will quite literally come a stop between shifts. Not, sort of. It will flat stop and roll backwards. Just between shifts. A decent company driver will make appropriate headway and lose no more speed than necessary. The O/O will make the split-second shift in such a way that
- speed
- temperatures
- drivetrain longevity
- fuel economy
are all optimized. It is not just the difference between a 13 or 18 speed transmission and a 9 or 10 in a company truck (it is partly) but taking in all the information of road type and surface condition, load, weather, altitude, traffic, etc. You may not have these options in your Tonka toy, but understand that
each aspect or component bearing on efficient operation is exhaustively studied in this industry. The O/O has only himself to rely upon, not the subsidies and cheaper financing available to fleets.
The above is better stated for a commercial driver, and who's to say he won't come around here and correct me? But the RV owner ought to be able to come to his own understanding of this minutiae. What's a matter of near-irrelevancy in a car is a
distinct percentage loss in a heavy vehicle.
The aids to determining
best adaptations to present conditions are the well-known Scangauge (if available; big trucks have their engine manufacturer versions), and an EGT and MAP gauges for starters. As well, a fuel flow meter (but these are expensive by comparison), such as
FLOSCAN. As with the above O/O example, comprehensive temperature and pressure monitoring of all systems/components (is also expensive).
As I see it the point -- as before -- is to
not fall below a favored fuel burn rate: where speed/time is not the determinant of headway (after safety/reliability/longevity concerns are met). Gauges other than airspeed are to help grease the skids towards highest numbers when desired, but to
absolutely (for that day's road/load/conditions) not fall below the desired mpg. Their expense makes them unlikely for an RV, but tracking (logging) what is available in some readable format is the homework I think you are looking for. The number crunching needs more than just the basic inputs.
The "desired mpg" goal is lower than the highest number achieved (racetrack numbers), but it is proven viable over a calendar year of driving (time/temperatures/altitudes/scale weights/towing). You need first to determine what it will be. And how much is skill versus rig condition (aero/rolling problems).
The question is, how much do you speed up without having the increased rpm level cancel out your advantage of the tail wind, any idea?
Rolling resistance is the most important consideration up to 55-mph:
no steering slop (poly bushings on anti-roll bars, etc), perfect alignment, Centramatic wheel balancers; best shock absorbers; no brake drag, best tire choice and pressures (TPMS system; numbers per builder and scale weights). One wants the vehicle to be loose and tight as appropriate.
Sensitivity is vital, IMO.
Aerodynamic: Coolant system like new (thermostat not only opens, but opens fully; no mineral build); exhaust system has minimized backpressure (tested); if turbocharged, CAC system verified.
The motor has a nice and wide envelope so that any speed changes don't start to track on the appropriate gauges as quickly.
It is all about predicting fuel use.
There's no way around the 55 thing. I like 58 'cause it just feels that much faster. 62 might be your upper limit. The calculation of winds might allow a scooch more.
As to gear splitters, what I read on Gear Vendor and USGear is that the latter is "better". I'd snap up that $800 piece if at all possible. What I otherwise "know" about them both is that they are not standing up to use by the hotshot crowd when deadheading (empty miles); or have found to not add a sufficient increase to warrant initial and ongoing expenditures.
Racers care about that aspect not at all. So, shall you be a racer or a trucker? It's a different philosophical approach. One is to be the best, damn the cost; the other is to have the lowest overall cost of ownership & operation. Both travel the same road a long distance together, but make a serious route change from the other at a particular point.
For example,
Aerohead is a racer with that cool rig of his. Once proven, who knows what step he'll take next? But he won't be carrying multiple 3,500-lb loads in that Bonnevile-bound truck and trailer (is the salient point.)
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