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well when driving my brick(67 GMC truck) I have to say I don't notice much difference in mileage between third gear and OD( yes I have a period correct borg warner 3 speed with a 30% oD). The difference I do notice is in the vacumn the engine runs in and out of OD. In 3rd gear I am usually pulling 15-16 inches of vacumn at highway speed(about 2400 rpm , in OD abnout 10 inches of vacumn( about 16-1700 rpm). I am guessing my 3.5 to 1 rear gear isn't low enough to effectively use the OD.
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