Yes, we are not getting all the facts here.
There is no way for the engine RPM to stay the same from 70 to 40 MPH in the same gear. The Allison transmission torque converter is locked up in all higher gears. The amount of "slip" needed to fit your 70-40 would indicate a destroyed transmission-- very unlikely.
So first job is to get the tach working.
Then, PLEASE tell us the lowest RPM in each gear before the transmission downshifts when at wide open throttle.
And, as already asked, check the transmission for diagnostic codes.
