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The units are inches of mercury, usually abbreviated In Hg.
It's possible that the timing is significantly retarded at 1500 RPM compared to 2100 RPM for emissions purposes. In that case, the torque COULD be less for the same fuel burned. If so, the higher speed should get better mileage. Maybe. You would need to drive one way for a tank, the other way for the next tank, and repeat until you saw a clear pattern to know for sure.
The largest benefit of a vacuum gauge is as a guide to minimize the range from the lowest to highest vacuum that you use when driving a particular route. The steadier you control the gas pedal, the better your gas mileage. Except for accelerating, sometimes you are better off to just step on it and go.
If you can borrow a portable fuel air ratio meter, find the fuel air ratio vs vacuum. Rich mixtures give a little more power by burning a lot more fuel. Then try to stay below the fuel enrichment vacuum.
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06 Canyon: The vacuum gauge plus wheel covers helped increase summer 2015 mileage to 38.5 MPG, while summer 2016 mileage was 38.6 MPG without the wheel covers. Drove 33,021 miles 2016-2018 at 35.00 MPG.
22 Maverick: Summer 2022 burned 62.74 gallons in 3145.1 miles for 50.1 MPG. Winter 2023-2024 - 2416.7 miles, 58.66 gallons for 41 MPG.
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