Wont Hold a Gear on Highway

[00ranger30]

I have a 2000 Ranger 4x4. 3.0 V6 with 4 speed auto tranny and 3.73 rear axle ratio. Everything is stock except for 31 inch A/T tires and the throttle cable mod. Going down the highway at 60mph the truck will not hold a gear. Its constantly shifting between what I assume is overdrive(4th gear) and 3rd gear? At 60mph even the slightest pressing or releasing of the gas pedal will make it shift. Its pretty annoying and I imagine causing premature wear on the transmission. Any ideas? Would turning overdrive off help or could it be harmful?

 

[RonD]

Won't hurt it, 3rd is 1:1 ratio, until recently that was top gear in automatic or manual trans
OD is .75:1 so lower RPM at higher speed(for better MPG) but little or no torque.

If trans was having an electronic problem the OD light should flash.
And it may have some transmission codes you can check even with no CEL or flashing OD light.
Transmission shop can usually read the codes, write them down and post them here.

The 31" tires throws off the speedometer and computer uses speed, RPM and engine load to calculate best gear to use.
3.73 axle in OD with 31" tires at 60MPH would be about 1,800 rpm
28" tires would be at 2,000 rpm

 

[4x4junkie]

Most likely it's the tires.
As said above, the larger diameter messes up the shift points. Solutions are: change your axle gear ratios to better match the tire size (4.56:1 would be best for 31s on a 3.0L), or switch back to the stock size tires.

Welcome to TRS

 

[pjtoledo]

that's normal for a 3.0 of that configuration.

My 2000 same as yours has always had problems holding freeway speeds, slight up hills or strong headwinds will cause constant downshifting. that was with factory 235/75-15 tires. had slightly larger 245/70-16 tires on it for a while, problem was much worse. when I had 225/70-15, a smaller tire, it would cruise with no problem. the 3.0 just doesn't have enough power to push a 4x4 auto very well. It does seem a bit too eager to downshift. if it wouldn't downshift until almost WOT driveability would be improved.
for a temporary fix try turning overdrive off and see how that works out.
the owners manual states to turn overdrive off when towing, so no harm should be done.
However,,, turning overdrive off also keeps the torque converter locked up during deceleration so coasting will be affected. that provides engine braking, very helpful when towing.

Perry