What kind of mileage are you getting on the highway? We converted a 300SD and it was getting about 27-29 highway with a 2.47 ratio. Those must be some serious hills your climbing....ours had no issues with that and it even had the lower stall (1600rpm) torque converter. 3rd is easily good to 50-55mph for normal cruising without issues with the 2.47 diff. On the highway it was around 2400-2500rpm at 70-75mph, no issues with hills of normal size.

My car is the opposite, I have the 3.07 diff with high stall torque converter, its a torque rocket ship in the city, but does high rpms on the highway of course...

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