My POV (nothing unique I suspect).
If we simplify to remove all other variations. I think that's the 1st (ideal) case (Peak Energy Efficiency BSFC up hill (which not always the same as WOT) , EOC down hill, same average speed/conditions,etc).
The YMMV comes in (I suspect) from when some of the other variables start changing as well. For example.
- If to get the power to accelerate up hill you moved the engine/transmission into a less energy efficient condition. This is a negative reducing possible gains.
- The worse BSFC for the ICE is one part, and can be seen on a BSFC if you have such.
- Worse for transmission is not mapped as often as a ICE BSFC.
Although transmissions are usually very efficient (upper 90s %). All gears are usually not exactly equal in their throughput energy efficiency. Some will be a little bit more efficient than others. When the simple case of two gears being otherwise equally efficient, higher transmission RPMs are usually less energy efficient than lower transmission RPMs.- Also worth noting here, from 98% to 99% is a 1% difference in an absolute sense, but it is also double the losses (1% x2 = 2%). As such it might not always equal to a 1% difference in observed overall fuel efficiency.
- Any air speed variation is always less aerodynamically energy efficient than an equal average wind speed. This is due to the exponential nature of wind resistance, and does not apply to any types of linear resistance.
- For example:
Person A's speed is steady relative to the air.
Person B's speed varies relative to the air.
If both average the same overall speed, and if all else were equal.
The speed variations of person B would require more energy.. Thus less efficient.
This is a negative acting to reduce potential gains.
- Speed creep.
- Many (not all) people get desensitized to faster speeds. The slower speeds seem slower than they really are. This can lead to the person who is accelerating having a subconscious leaning toward less time at slower speeds. Increasing the overall net average speed. If than one averages a faster speed they will have more net wind resistance energy losses, and thus lower combined MPG.
- This also happens with some people with a positive psychological feedback to the psychic effects of acceleration itself. If they perceive the acceleration to be a positive thing, some people will then subconsciously do more of it. To people of that type the gravitation effect of going up hill adds to the feeling of road acceleration.
- For any newbie who might has asked:
Why not maintain peak BSFC energy efficiency on both up and down hill .. because then you'd be going much faster and have much higher wind resistance losses. The faster you go the more energy per mile you are spending. If ICE and transmission is already at peak energy efficiency, any speed faster than that is only a negative.- There would be a potential overall breaking even point if there is another hill to climb after the down hill. Exact break even point would vary. But, as such , under some conditions, staying at Peak Energy Efficiency ICE & Transmission might be a net overall benefit. Although the continued running of ICE on the down hill, will (almost always) be a negative for that one hill.
- Worth noting that higher fuel efficiency is possible at lower energy efficiency. If you go slower your energy losses/(required consumption) per mile go down. As long as those energy consumers go down faster than one's fuel to wheel energy efficiency you are getting higher Fuel Efficiency , even with lower Energy Efficiency.
- Changes in the air (Up-hill vs down-hill)
Uphill and down hill wind speeds are almost never the same. Nor is the air pressure. Nor is the air density. etc. One's wind resistance is not about vehicle to ground, but vehicle to air. - Although of course not true in all individual cases/conditions. I suspect overall for a large enough sample size. That statistically one side has higher wind speeds , higher air density, etc than the other side of the hill. I further suspect that (in a large enough sample size) it is the side that is up-hill from the POV of the wind that is most often the one with faster,denser air speeds.
- So, going faster up hill statistically most often faces a relative air benefit. Although likely to be tiny.
For Example:
Say The Wind is moving West to East.- Those going up-hill (driving west to east) have the wind at their backs and their relative air speed is lower, thus less wind resistance , thus less energy to travel the same speed. Both Uphill and down hill. But the Air itself as slowed a little and provides less help, thus more wind resistance on the down hill side.
- Same hill same wind direction. The air/wind (statically large enough sample size) has lost a tiny bit of density/pressure/speed as it climbed the hill, and now requires slightly less power/energy for the car against it driving up hill. Those heading East to west are driving into the wind (both uphill and down hill). Up hill face slightly less dense air that is slightly slower. While they will face slightly faster and slightly more dense air on the down hill.