I had a friend tell me that my trouble with my transmisson was that it was slipping. With OD off it slips in the last gear. But it works fine shifting through the other gears and in OD. I'm no expert on transmissons. So could this happen? Also my f-150 has a 10" lift kit with 38" tires (I know it's hard on the trans) but is there another "beefer" transmisson I could put in or is there something I can have done (when rebuilt) to help handle the extra stress? Thanks!

 

You need to leave more specific info:

-Which trans code "U" or "E"???
-Gear Ratio???
-Did you change the speedo gear when you increased the tire size???

Also: I'm not so sure that the transmission is actually slipping..... With 38" tires, it may just be that the engine is not producing enough torque to keep the torque converter in lock-up.

 

My trans code is "U". I almost positive that I have 3.53 gears. And I did not change the speedo gear. As for torque I have add a computer chip, headers, K&N air filter and also took the air flow restr. out of the intake housing (thanks to F-150 online). I figured that this must have boosted it some. I have had the truck jacked for over a year and have had no problems before. I really don't believe it's slipping either, but everyone one else thinks so. Hope this will give you more info. Thanks for any advice or help you can give me. It's driving me CRAZY!!

 

You have a 4R70W Transmission.

First: You need to change the speedo gear. 3.53 is not a ratio I've ever heard of, so I'd look into that. If you really have an actual 38" tire, with 3.53 gears, the computer will not read this properly and will not be able to shift accordingly.

This thread has links to good tire size a speedo gear calculators:
https://www.f150online.com/forums/sh...threadid=98075

I'd suggest that you first change to an accurate speedo gear and then consider flushing the trasnmission and replacing the fluid with Mercon V.

The speedo gear is only a few dollars and may save you from a transmission rebuild. I don't know how optimistic you should be about this though. A 38" tire is HUGE!!! Your transmission may be worn out from this.

 

Thanks for your help AjRagno!! I might have 3.56 gears? I can't remember right of hand. I'll check into that. One more question, would the transmisson just all of a sudden start doing this? I have had no problems in the past. But I will be sure to change the speedo gear even if I have to replace the transmisson. Thanks again for any more advice you can give me!