depends on a bunch of factors. What car, what road, what wind direction and speed, how much stop and go, what tire inflation? During the 55 years, it was figured they could have saved more gas if they had keep the speed limit at 70 and mandated 4 more psi in all tires. My SDL got better mpg at 75-80. Found that out when I was making almost daily 200 mile round trips when mom was sick. Went from 23 to 27 mpg. My old '74 Datsun 260Z barely got 20mpg at 55, but got 26.5 at 85mph. Whole lotta factors involved...but that doesn't fit in the safety nazi or car haters agenda.