how It's
It is a very unusual car that gets better fuel economy at 65 than at 55. And there are no unmodified production cars that get better fuel economy at 75 than at 45. While it might be true that your engine is more efficient when running at the rpms needed to go 75, it it is very unlikely that it so much more efficient that it makes up for the extra power required to go 75 versus 45. I am not sure exactly what car you own, but unless it is very small and very aerodynamic (the Honda Insight is about as small and areodynamic as a production car gets these days), I am confident that if you drove exactly the same route under exactly the same conditions at various steady state speeds, your car would most likely get it's highest fuel economy at somewhere around 45 mph. People often think that their cars get better mileage at higer speeds becasue they never drive on the open road at lower speeds and they compare their constant speed 75 mph highway mileage to a stop and go 45 mph trip on a busy street or secondary road with lots of traffic and stops.
.
Ed
