Also if you upgrade tranny to a torrington bearing internals instead of just factory shims. It frees up the horsepwr a little. We are not talking giant difference but it helps take some of the heat out of tranny. I run a lowgear kit and all the upgrades and can tell a difference but slight noticable. On the torque converter I've been running a hughes torque converter and had some really good luck. haven't broke it yet!!!knock on wood!! The mileage will come from the type of torque converter you run and www.hughesperformance.com. On a 4speed you sometimes rev the engine up and this uses fuel. on a auto tranny this doesn't happen, unless you run a manual valve body. also their is a valve body you can do both and keep the pump pressure up when manually shifting. Their are different options but how much do you want to spend. I ran TCI converters but had to many failures in this application i was in. I quit them and been hughes ever since. drop a message if any questions. Their are differences in c-6 cases so some are better than others. Some of the older cases had some issues and had better luck with the newer 1978 case and newer and the cases with the ribs on back of case are a little stouter also. lost in wyoming and lovin it!! Forgot to mention my driveline issues went away once i went to a automatic. Easier on a u-joint compared to the occasional 4speed dump the clutch day.
Do you know what your on-road, before & after mileage figures were?
I know I can tell the engine runs easier and less heat in tranny according to temp gage on hot summer day. I run a lower stall converter for pullin the mtns in wyo around 1600. hughes built up for me. didn't want a lot of stall. My ford is running on 40" hawgs and get around 9 to 10mpg on interstate. If running 38.5x11x16 boggers can get over 10mpg. I think this is good for a big truck. motor is solid cammed for lowend and midrange not really a high rpm situation just pullin mtns and huntin rig. Its a extreme huntin rig but it gets the job done. Pulled my dads new truck out again this year because the weather came in on us in mtns with rain/snow and sleet hail and whatever else mother nature could throw at us.