This is for my son. He has a 2004 psd 4x4 raised w/373 gears and 325 tires. The problem is he just brought a 40ft. toy hauler. Is there a chart or way to determind his actual gear ratio. He wants mimium 4.10 to 4.33 to pull his toy hauler better. I'm beting he has 3.50 or so with those tall tires. Please help, Chet

 

You might want to put this question into:
6.0L Power Stroke Diesel - Ford Truck Enthusiasts Forums

 

Not quiet sure that's necessary but you would have twice the chance for some beneficial info. To the OP if he's wanting to do a gear swap and continue running those big tires I would go with the 4:56 gears or something close to that. But if he ever plans on running some smaller tires he will be stuck turning some extra rpm's and possibly sacrificing some fuel economy.

 

The difference in his tire size would change the ratio ~ 10%... give or take a % based on actual tire diameter. He is closer to 3.36 right now. Based on a 10% difference, 4.10 would give you ~3.69 and 4.33 would give you ~3.90. The difference isn't that much, each way, with either gear. 4.56 yields a 4.10 ratio.
I am running 285's with a 3.73 gear and my ratio changes to 3.58, or ~ 4% difference. I haven't seen a noticable power loss with the slighly bigger tires and pull a 35 ft high profile 5ver with my current mods.

 

Thanks Bulla and Roland. This helps. If 10% is the ratio he has a better chance to pick what he wants. His weekend warrior is model 3905 and he says its 41 foot long. We measured the height and it is 13'5" to salite top. He noticed the difference as soon as he pulled it out. He had a 30' toy hauler and this one weighs as much empty as the other one did fully loaded. Its nice but I tryed to they him. The truck is 4x4 so he has to buy both sets of gears. I would like to see him in a 550 but it is what it is now. God Bless, Chet

 

Chet, here's a link to a chart identifying ratios and expected changes. it may help him decide.
4Lo.com :: Gear Ratio & Tire Size Chart

Barney

 

Thanks Barney. I'm trying to figure it out. This helps alot.
Chet