Drivetrain that is. Donna has a C4 transmission. That is an automatic with 3 forward gears and 1 reverse gear. When we rebuilt the rear-end, we put in 3.00 gears instead of the original 2.73. I could show you the math, but the short story is that at 70 mph Donna will be running over 3000 rpm. Not very comfortable or fuel efficient.
There are two main solutions to this problem. The first is to replace the C4 transmission with an Ford AOD transmission. The AOD is an automatic transmission with an overdrive gear, effectively making it a 4-speed. The overdrive is a 33% gear reduction meaning we would be turning around 2000 rpm at 70 mph. Much better. There are several concerns with this option. One is that the AOD is not a direct fit replacement. The AOD was introduced in 1980, a decade after Donna was built. The other is that several connections and mounting points are different. Some people have good experiences, but others do not.
The other solution is a gear-splitting tail-housing or overdrive. This is a replacement for the original tail-housing that has an overdrive unit built into it. This would retain the original transmission that Donna was born with and add the overdrive without making any changes to the linkage or connections for the transmission. However, we will have to shorten the driveshaft (or buy a shorter one). Also, some people who have installed these units complain about vibration and/or noise, others do not. It seems to be related to making sure the angle between the overdrive unit and the drive shaft is correct.
This chart shows the Transmission ratios for the C4 and the AOD, plus the ratios for the C4 with the gear-splitting tail-housing.
With the tail-housing solution you get options for in-between gears (in manual mode) and a 2.34 final ratio, moving 3000 rpm down to 2340. With the AOD you forgo the potential for gear splitting through all forward gears, but get a little better final ratio of 2.01, moving 3000 rpm down to 2010.
So, the choice is between an add-on to Donna’s original C4 transmission that would give us a 2.34 final ratio, or a replacement AOD transmission that would give us a 2.01 final ratio. The C4 is more robust than the AOD. However, ours stills needs rebuilt and if we go the AOD route we would just trade it for the core charge and buy a rebuilt AOD with a warranty. Decisions, decisions….