Need Info on Best Towing Pkg.

[wolfhound]

I Have a 1989 Chevy Duelly X Cab, 454 CID (Injected) 3 spd Auto. I'm running a Gear Vendors Over/Under drive @ 1.3:1 and a 4.10:1 final drive.

The engine must be in good condition because it passes emissions with such low values the inspectors can't keep their grubby mits off the under hood connections.

When I'm towing I don't use the gear vendors unit and 3rd gear in the auto trans is 1:1 but the truck just seems like it's running out of it's torque range and the 1.3: 1 overdrive seem to be to much.

Is there a source of information out there that will optimise the Eng / Trans / final drive ratios for best results?