BobK The rpm at 75 is right at 2500. Don't rem what it is at 55. I think I need to say that the 55 really has nothing to do with my question. I just know I get better mileage at 75 then any speed. Car has single exhaust, No cats, CIS injection, No computer controlled mixture. I have logged all fuel and mileage for the last 8 years and can see that the best mileage is when on long interstate trips.

Well, time to close this thread. I've been told in so many words that I don't know what I am talking about yet my numbers tell me otherwise. I believe my numbers and others can believe what ever they want.

But thank you BobK. I think you have hit on a possibility of efficiency at an RPM. I guess my car just likes 2500 rpm.

Paul

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84 500 SEL (307,xxx miles)