The last time I got on the highway, my truck only shifted in 4th, and did not go into OD, or lockup like it used to. I had taken out the thermostat, and it ran below op temp. for a while, but now it's summer, and running 180 or so. I was told the motor had to be a certain temp for it to lock up. But it still does not. I get horrible gas mileage, needless to say. My main concern is whether or not I am damaging or shortening the transmission life by running 65-70 MPH in 4th gear.

 

So it's the opposite. I should not be trying to do 65 in 4th gear. I stay in town with the old beater now anyway. Something else....is it a rumor or is it true that if I change out fluid that hasn't been changed in years (transmission) that it may quit working? Is that a "if it aint broke don't fix it" item for the driveway mechanic?