I used to use a vacuum gauge back in college on my 87 Celica, so I can relate to this question. Although it depends on if you are driving in the city or high way. I used to use the hypermiling techniques before I read about them. Driving with load or rolling to an intersection without stopping.
What looked at was the engine dyno for the engine in my car which was a 3S-FE Toyota engine. It had a pretty flat torque across the RPM range. Thus I shifted at a lower RPM. In addition, it made sense to me because for most calculations regarding force the velocity is squared.
BTW the vacuum gauge I mostly used on the open road or highways because it is very sensitive to the gas petal input.
Just my 2 cents.
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