I had no change but I did it backwards. Went from OD to no Od.

87 Ram D100 reg cab shortie. Swapped in a 5.9L Magnum and a 518. With 3.55 gear and a 28" tire.

I did not drive for mileage. I did much of my driving at a comfortable 120km/hr.

I lost overdrive but continued to drive the truck in 3rd gear. Except now 120 km/hr was too much rpm so I cruised the right lane at 105 which was a little high in the rpm but with the reduced speed I was still getting 18-19mpg.

After losing all confidence in the 518, I swapped in a NV3500 5 speed. Then I was finally had a mileage improvement.