I put in a gearvendors OD last winter , so far I have to say it was well worth it

What kind of ratio's does the GV have and what speeds are you driving normally?
Reason for asking is I have a installed a A518 OD in my '73 Dart and don't come close to the rpm you are still running on the highway, even with my 3.91 gears in the rearaxle.