I have an 89 F150 with the 4.9L I6 2WD. Originally it had an Automatic transmission but at some point in its life it was switched to a M5OD. I noticed that the overdrive feature doesn't work the way it should. I have seen in other places where if the gearing in the diff is too low or too high (can't remember which) it could cause the overdrive to not engage. What rear-end came paired with the M5OD R2?

 

All of the f150s from this era came with an 8.8 rear end. But ratios for it were not specific to transmission. If you get the axle code from your vin tag, you can get the ratio. And what do you mean when you say that overdrive "wont engage", are you unable to put the gear shifter into 5th gear?

 

I know it has the 8.8 and I know the gear ratio that is in it now is the 3.08. But by not engaging I mean if I put it in 0D/5 on flat ground my speed drops.

 

3.08 is a pretty passive ratio. What speed do you usually shift into 5th gear in? And what rpm range does it put you at?

 

I've shifted any where from 35 to 55 and sometimes higher.
Rpm are 3000 to 3500

 

Those engine speeds seem high to me for the speeds you mention having a 3.08 final gear ratio.

 

I know on flat roads I'm at something like 3000 rpms going 60 mph in 4 but as soon as I go to OD/5 my speed drops and I have to downshift to keep get back up to speed but then on pegging my Rpms to do so.

 

That's not what you said in your previous post when you said you shift at 35-55 with engine speeds of 3000-3500.

 

I may be mistaken on one or the other but will get accurate numbers after bit

 

Okay so i did a couple of test runs and the shifts are 1-2 is 25 MPH at 3500 RPM, 2-3 40 at 3500, 3-4 50 at 3000 or 57 at 3500, 4-5/OD 70 at 3000 but as soon as i put it in 5th RPMs are at 2500 and then it falls flat and speed starts dropping with RPMs unless going down hill of corse. And these are me shifting for trying to get up to highway speeds.