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The lowest RPM in the highest gear with the engine in lean-burn gets me the best mileage.
If I'm on a long stretch of road I'd rather be going 50+ mph
1000rpm is 30mph and it gets over 100mpg but you lose lean-burn if it drops below 1000rpm and there isn't much reserve torque for anything but a completely flat road.
On a warm or hot day with windows closed and air conditioning compressor off
50mph, 1700rpm, 90mpg
58mph, 2000rpm, 85mpg
64mph, 2200rpm, 80mpg
70mph, 2400rpm, 75mpg
70-78mph can be acheived under lean-burn which has a 65-70mpg(lower at lower altitudes and if you are near 10,000 feet you might see 80mpg as the limit but with less air drag I've cruised in the 75-78mph range at 80mpg on a high density altitude(hot and high altitude, no AC) day) can be achieved on perfectly flat roads and after 78mph the car typically will drop out of lean burn when VTEC kicks in when you are above 2700rpm.
The brake specific fuel consumption is a smaller factor than air drag in a car and for the engine, you want the throttle plate as far open as possible without triggering fuel enrichment, which usually means the lowest engine speed that can accomplish the task. The Honda Insight does this with lean-burn to reduce the pumping losses and keeping the engine at roughly 80-90% load at a low RPM relative to its smaller engine displacement and power output capability. Cars with CVTs can target, although they might not necessarily do this or do it well depending on the CVT, for a specific engine load or throttle position to maximize throttle opening while keeping the engine at the lowest speed possible. If you can achieve a high enough engine load to where the engine couldn't produce more power through throttle opening and there isn't much excess power, you are now in a position where it's better to hold that load than to do pulse and glide(engine-off).
Most engines are overpowered or their transmissions won't bring the engines to a low enough RPM for this to be the result and we need to use engine-off coasting to drive with enough load to add efficiency.
In my Insight, if I use a speed higher than 40mph, I might as well use lean-burn, anything below that and pulse and glide makes sense, but I don't care anymore because gas is not above $3/gallon and my driving below those speeds without traffic behind me is far more uncommon with the driving I do today(pre-COVID) than it used to be. ..now I'm just barely driving at this point
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