I have been around the block on this and have THIS to report:
High stall converters and taller (Lower numerically) gearing do result in excessive slippage, sorta negating the advantage of the taller gearing. I ran a Gear Vendors overdrive with a 3.91 gear, 28" tire and a slippy 9.75 converter. Not fun. The car felt lazy at freeway speeds, like a boat in the water in terms of part throttle feel. The combo was fine for wide open throttle, it just sucked for everything else. I don't care to drag race, I enjoy distance drives, cruises, cornering on twisty roads and such.
I eventually sold the GV and went with a 3.55 gear and an 11" converter that made a huge difference in how the car feels. I really like this combination.
I really have to reply to this as I am the member that bought the G/V from Frankenduster. Let me say right here that he is a stand up guy, fair and honest and I love his car. I have a 4,175lb convertible 66 Coronet with a 540" HEMI. 4.5" stroke, 246@ .050 I roller cam and 3.54 Dana. Torque is not a problem with this set up! Last fall, I drove the car almost 400 miles round trip to the Mid TN Mopar show in Lebanon TN from Chattanooga. Most of the trip was Interstate at 70+ MPH. 3,300 rpm with semi trucks on your bumper is NOT fun! The car had a 9.5" converter and would still "slip" 300 RPM going up a hill. With the G/V I installed a Hughes "2,800" 11" converter. Now 70 MPH is 2,500 RPM on the flat and level. It still gains 200-300 RPM on a hill. But it is SO much more pleasant to drive on a trip! In an hour drive, the noise, vibration etc. is VERY reduced. With the 640+ ft.lb. of torque, I should have sprung for the TCI "power adder" converter for $900 with a stall of 2,200 RPM but couldn't afford it at the time. The OD ratio ends up at 2.76 and the car is just a joy to drive @ 60-70 MPH. In short, if you have the torque, you just don't NEED deep gearing, I like my "rear end" just fine, don't you?
