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Throttle position and load are two different things. Manifold vacuum determines load, higher vacuum=lower load. When you transition from downhill to uphill the load increases beyond the percentage of throttle position. Efficiency is measured by the actual compression in the cylinder when combustion begins is directly related to manifold vacuum and can vary significantly with very small changes in throttle position.
My Insight had an instant economy gauge which I used to refine my acceleration technique by accelerating with the lowest loss in instant MPG while providing an increase in speed which could then be used for a period of coasting. The Insight would coast in neutral at 150 (max measurement) in instant MPG down to 16 MPH which indicated and idle consumption of .11 GPH. When coasting at multiples of 16 MPH the instant MPG would be 150Xspeed divided by 16, so at 64 MPH the instant MPG would be 600 MPG. Using this on slight grades I could average over 90 MPG in the Insight and it was with a CVT transmission.
If you go by throttle position then at a point in load the AT will downshift and you will suffer a dramatid loss in instant MPG. Without precision instrumentation the best technique is to avoid any downshifting in a auto transmission to the point where you are loosing to much speed (when climbing uphill). This is where you would wantt to consider allowing your speed to slowly drop in order to maintain top gear. The same also works with manuals. Avoid downshifting to minimize the loss in MPG climbing the hill, but not to the point where your speed drops below about 75-80% of your desired average or you take so much longer to get to the top of the hill at lower instant MPG.
I will even accelerate before reaching the bottom of the hill, if I risk to long on the climb.
As I said before it really depends on the hill and you strategy changes as the grades increase, but if you do it correctly you should do fine until the downhill portion does not allow you to coast without engine braking or any use of friction brakes. Only when the grades get beyond that percentage will you suffer loss of average MPG.
I call it the roller coaster scenario. The coaster climbs the grade initially and from that point onward it is all inertia. The distance the coaster travels is many times greater coasting than the initial climb. I remember one member here telling about coasting 35 miles coming out of the Rocky mountains. My longest coast has been about 3.5 miles.
That was also enhanced by drafting on I85 coming east from Blacksburg Va. I had to used engine braking, in my Insight and the battery charge dropped dramatically on the lcimb but recovered completely on the downhills. I average 70 MPG with an average speed of 55 MPH in the Insight on that trip. I think it was 635 miles on 9.65 gallons of fuel.
regards
Mech
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