Overdrive Options

This is an interesting site that backs up Tjfly4pay's experience:

http://smrtrans.tripod.com/smrtransmissionsintro/id21.html

They say:
"That A500 or A518 transmission in your Mopar is consuming enough horsepower over one of our fully prepared 904s or 727 to cost you up to 1 1/2 SECONDS in Elapsed Time in the Quarter Mile.
NOTE: IN A 3600 lb VEHICLE YOU WOULD HAVE TO ADD 100 HP TO DROP 1.5 SECONDS.
Such power consumption is a result of the very high inertia weight created by the components required in the overdrive portion of the transmission. These components are stopped until your truck starts to move. At this point they must accellerate from a stop to full driveshaft speed."

Also; interestingly, they claim that the Gear Vendors unit is much more efficient than the A518:

"To obtain the MAXIMUM PERFORMANCE potential of your truck you will be losing the rpm reducing potential of your overdrive gear.
If this function is important to you but you still want a major performance improvement, we can provide the GEAR VENDORS overdrive unit to the conversion kit. Installing the GearVendors Overdrive will add approximately 1/4 second in e.t."

Makes me wonder what is the main cause. Is it excessive hydraulic drag or just mechanical inertia? Sounds like it is a lot of both.