Your current gas mileage for a 25 year old gasoline big block design does not seem outlandish at all. What is your mileage at 55 MPH? -- driving faster really drinks the fuel rapidly as the air resistance goes up non-linearly with speed.
Do you have the factory recommended tire sizes and running at the proper inflation?? Have you put on a scale, and VERIFIED that it is not overweight??
If you have an accurate tachometer, what is the engine RPM at each of those speeds, as that might indicate if any optional gearing is involved here...
Chuck
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2007 Tour Master T40C
Acura MDX toad
"It takes a great deal of time to recover from any improvement..."
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