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You are correct,
At lower speeds, MPG gains are greater than at higher speeds.
The MPG gains are in line with the torque.
Most of the time reducing torque by 10% results in 10% better MPG, or, accelerating with 10% more torque results in 10% lower MPG.
Out of several hundred readouts, you'll find that MPG roughly is inversely proportional to torque, which torque is directly proportional to gearing.
But with a 10% torque decrease (aka 10% lower RPM), you may see upto 20% MPG increase at lower RPMs (eg: 3k RPM), while see much less at top speeds.
I guess it also depends from machine to machine.
A bike or car that is undergeared (eg: does 4k RPM at 40MPH, with a 2 liter engine) will gain 50% MPG when lowering RPM to 3k RPM.
A small 250cc engine that is very taxed, doing 6k RPM at 80MPH, will most likely gain only 1 or 2 MPG when upgearing it by 10%.
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