Gary explained it really well: "The ratio of the tranny stays the same with or without the lockup, but without it, you get slippage so that final ratio is compromised by the percentage of slip as the engine must turn that much faster. So, actual ratio stays the same, but effective ratio gets worse and you lose some of the overdrive advantage". I run a 200-4R and I have the Lockup. I am running a 9.5" billet 2800 stall LU converter, but I still get a 200-300 rpm drop when I lockup. I get a 300 rpm drop when I am partly in the throttle and only 100-200 rpms when at cruise and it locks up. I don't use LU when going to WOT because I am only running a single disc lockup. I have mine setup with a module that allows 4th gear to engage without lockup...the module waits for 8 seconds after 4th gear comes on and if there is no significant throttle change in that time, it locks up. So I can feel 4th gear engage and then 8 seconds later I feel the lockup engage... it does in fact feel like a 5th gear. It works very well. I also have a disable switch on the dash so I can turn off the automated lockup if I want to cruise around in 4th gear. At speeds like 45 the lockup isn't always needed, but being able to use 4th gear without lockup is nice in those situations. I had considered setting up the 3rd gear with lockup as well, using a similar switch, but I decided that had limited value, if any, so I never followed through.