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No. There is no speed* such that going slower will decrease mileage.

* Unless we're talking about some sort of pathological edge case in a vehicle without a properly specced transmission or something, or driving an automatic so slow it's just idling against the torque converter or something.

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No. I checked multiple sources with actual test results. Most cars get peak MPG at over 25 MPH.

I guess for modern vehicles though you want to be in top gear. Otherwise it’s more revolutions of the engine for a given distance. So the minimum speed to get into top gear.

Revolutions and fuel consumption don't really correlate. It's not like the engine injects a fixed amount of fuel every cycle.

When acceleration isn't happening (i.e travelling at a constant speed under constant load), it largely does correlate.

It's not a straight line. Brake Specfic Fuel Consumption (gal/hp-hr) is often higher in the lower 3rd of the rev band. Across the bulk of the RPM range the BFSC curves are basically flat for most engine. Throttle position matters much more than RPM.

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