I thinks is worst with the auto... the 5th gear on the 5 speed is a .856 and with the Over drive it is .688 I am not sure if you have the same final drive ratio but if so its likly just dont have the power at that low rpms to hold 70mph. I have had 2 carbed 2.5 both 5 speed they would go 70mph might be a bit more rpms then I liked but it was not that bad I have towed a fair almount with the 2.6 carbed. I also have had a turboed 2.6 montero that ran great! I all so tryed the 2.6 with both TBI and MPI N/A. They both made better power over the carbed motor. I have not had an auto tho.