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It depends on the car/truck, but my 454SS pickup got 14 mpg at 70 mph which was right in the perfect rpm range for the combo but would get 12-13 at lower speeds. The F-body get best gas mileage at 70-75 mph as well. 55 mph in an F-body is too low in the rpm range and will hurt gas mileage. A couple of my old 4Runners got best mileage in the 65-70 mph range so I don't think this 55 mph thing is accurate. With overdrive, almost all the new cars are geared for best cruising rpm around 65-70.
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69 Z28 JL8 4 wheel disc brakes - being restored
09 Silverado Z71
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