Hey hotroddave how low do you figure the torque peak on that motor is?
I have found with my 5.0 mustang that if I can only go 65 mph, it actually gets worse mileage in 5th gear than in 4th. This is with a .69 overdrive I think and a 3.08 rear-end. You said you were considering installing an overdrive. The 518 is a similar ratio and you have 2.73s. If I am going 80mph the mileage picks back up in 5th gear, but I have found when I spin under 2k rpm at cruise, or especially under 1800 rpm, the mileage drops off.
