Hi, Steve
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Originally Posted by Manualhybrid
empirically (without a scan-gauge), I found that a slightly higher speed than the lowest possible RPM speed in this condition produced a better result--45 mph or so yielded better fuel economy.
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I wonder if what you saw is the difference between optimal (flat road, constant speed) testing conditions vs. "real world" driving where it sometimes makes more sense to cruise a bit faster than the lowest speed at which top gear/lockup engages just to avoid automatic downshifts on hills, slight acceleration, etc.
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24.9 mph--which must occur prior to torque lock-up--is an even better efficiency speed!
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I've seen this in a few vehicles -- but only ones with automatic transmissions. Even though it isn't in top gear, the aero load is reduced enough at 40 km/h / 25 mph that it comes out ahead. If you could manually shift up to the next gear, the "dip" seen in the next 2 data points would disappear.
Darin