I have a '79 F150 4x4 Supercab with 4.11 rear end and 235/75-15 tires. According to my calculations it should rev around 2830 rpm at 60 mph. I installed a tach to check and it is revving 3350 rpm. 18% slip. Everything I've read suggests it should be more like 5%. I'm wondering if this is my fuel mileage problem. Previous owner said the tranny was rebuilt and the fluid looks and smells like new so I don't suspect worn clutch packs or brake bands. I notices one of the studs on the torque converter was stripped so I'm thinking it wasn't replaced and may very well be 32 years old. Stall seems to be around 1600-1800 rpm. Is there something else I should check before installing a new converter? I plan on verifying how accurate the tach is and double checking rear end ratio. Am I doing any damage to the rest of the tranny with a bad converter? Wondering if i could leave it til spring but will be using the truck lots. Fluid is holding up good. What stall speed would be best for stock 400? I'm thinking 1200-1500 would be nice. Really could use some advice!!! Thanks...Jack

 

Looks like I may have jumped the gun. Toyo says their 235/75-15 tires are 29.3" tall which is what I used to figure rpm. I measured my tires today, which are not Toyo's, and they were only 27.5". Turns out my rpm should have been 3000 at 60 mph. That and my other tach read 150 rpm less than the one in the truck. So now my slip is 7-12% depending what my actual rpm is. Don't really want to go buy another tach to figure this out so think I'm gonna drive it til the tranny goes out and then rebuild with a new torque converter.