Recently changed my 1993 cummins with an automatic and overdrive from 3:54 to 3:07,to help with highway speeds. I was able to do 70mph before which was on the governor and now when I’m on the governor I’m only doing 75 maybe 80. The taller gears made a big difference in slower speeds but once I get up to 60 it drops off on acceleration. I suspect stock torque converter is the problem. Recently flushed transmission it’s all clean and has no problems shifting. Truck has 265 miles any thoughts has anyone else had similar problems

 

Torque converter slip is probably a factor.

The 47rh (lockup) would do better, but have read that the rpm may end up too low to hold it.

 

Can you hold it in D longer before you shift into O/D?

I agree it is likely a slipping converter with the extra load from the gearing.

 

I can do 65 in 3rd without overdrive on, but I don’t think it’s an overdrive problem because it goes into overdrive at like 45 on the daily and seems to be fine, when floored from a stop the motor hugs 3k through all the shifts, transmission doesn’t really feel like it has any torque going through it.
looking into a converter from suncoast

 

There's a limit to how well a non-lockup converter will perform. It's a pretty big step from 1:1 to 0.69:1.

That's why they made the lockup auto trans for 94+, and there's always manual transmissions.

 

It sounds like the swap to 3.07 gears did exactly what it’s supposed to lower RPM at highway speed but it also exposed the limits of the stock torque converter and fueling setup. With taller gearing, the engine falls out of its optimal torque band around 55–65 mph, so acceleration feels flat even though top speed increased slightly, that’s common on first-gen Cummins trucks with factory converters that have relatively high stall and early lockup characteristics. A lower-stall or more efficient lock-up converter, along with checking cable adjustment and ensuring the lockup is actually engaging, can restore mid-range pull. Also confirm your governor spring and fuel plate settings are still stock, because with taller gears the engine needs a bit more fueling to maintain power under load. If you want to verify the exact transmission, axle ratio history, and factory specs before changing parts, running the vin decoder here https://carfast.express/en/vin_decoder can clarify what components the truck originally had and help you choose a converter that matches the drivetrain.