I have a 1991 ford explorer 4x4 all standard. With an automatic transmission. I travel everyday in the highway which is 70 mph. I mostly go 65 mph and my rpms stay at 3,000 rpms if I stay at that speed. 55 mph is 2,500 rpms. Is this normal? Increase speed rpms increase. 70 mph 3,500 rpms. I...