Hello everyone I hope you all had a great weekend! I know this question has been beat to death and I asked once but my situation has changed and I need some advice before I start spending money. I have a 2005 F-350 CCLB 4x4 with a 5.4 3v with 3.73 gears. I have a smaller dealership and I am hauling cars on my trailer 4 days a week about 4.5 hour trip each time. I used to have a two car I hauled but that's been replaced with my new tilt trailer. I'm running 20's with 35" tires and with a full size truck on the trailer it's shifting in and out of over drive for every tiny hill. Yes I am hitting the tow haul button. In overdrive I'm running around 1600 rpm maybe a little higher. I feel like I'm just lugging this little motor too much. And the constant shifting scares me on the longer few hundred mile trips. The only things I pull are a full size truck on trailer and once or twice a month my 10,000lb backhoe loader. This weekend I noticed a single clunk in the rear end that turned into bad noises by the time I got home. Since its time to rebuild the rear end I'm going to choose a new ratio. Will a 4.30 be sufficient or should I go to a 4.56 right away? I want the motor and tranny to have less strain but I still put on a lot of empty miles and would like to keep some mpg. My next set of tires will probably be 37's just to be a bit more proportionate. So that's my question in a nut shell 4.30 or 4.56. I hope I have enough info for you guys. I am not going to sell the truck and buy a diesel or a V10 since I know that answer is coming lol. Thanks for the help guys and have a great rest of your week!

 

35's- 4.56, 37's- 4.88. Maybe deeper gear than those, but that is a starting point to ponder.

 

5.4 running 35" tires and towing 4-5 days a week up at max tow capacity, have 4.88 gears installed and call it a day. Im on my phone so i cant see shere u live but if it's extremely hilly maybe even steeper than 4.88

 

Didnt read your entire post the first time, I see your going to 37's. With 4.88 and 37's the motor is going to be a dog towing, its going to downshift constantly towing that kind of weight. Stay at 35" tires and 4.88 gears, if you really want 37's make sure u get 5.38 gears. Even then u have a 5.4 motor turning 37" tires and towing 10K, good luck and post a video after the changes would love to see how the motor does

 

Do you have any idea what rpm I would run at 65?

 

Use this site, this is what i did when i had my 4.30 done

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