Gearing options

[Kurt]

Looking for some experience or opinions for gearing options. The truck is a 99 350 with the zf6 trans 4x4. Originally the truck had a 10" lift with 40s and 4.56 gears. Since then I have dropped to and 8" lift and currently running with a 38 tire. With the 40s Highway speeds weren't bad maybe turning 2000rpms at 70mph. With the the 38s at 70mph its almost 2300. Fuel mileage sucks, I drove across state and hand calculated 20mpg with the 40s, and over all acceleration isn't what it used to be either. So the 4.56 are to tall of a gear for the 38s.

Couple of choices, regear the axles, or go with a gear vendor overdrive unit. I hAve no experience with the gear vender unit and do t really have many videos on YouTube of real truck reviews. Not sure what to defeat to either. I know there are charts and what not but would like to get some input on these options. What do y'all think???

 

[01yellerCobra]

The 4.56's are short gears. Tall gears would be like 3.55's. Think of tall gears having longer legs on the freeway.

Anyway, I know guys that have run the Gear Vendors set up and have no issues. But you should compare prices between that and regearing the axles. I bet you'll be close. And personally I'd regear the axles.

 

[bcvickers]

You're going to want to go to probably 4.11's to get back to where you were at with the 40's. 2000 rpm at 70mph is the sweet spot for these trucks; too many more rpm's and fuel mileage drops like a rock, too many less and you're waiting on the turbo to spool anytime you get into it. That's how it works with stock engines anyway.

 

[patrickc]

My Dually has 4:10’s. 6.0L diesel, automatic, on 35’s. I run right around 2100 rpm at 70mph, in OD.

If you ask me 4:56’s are a little too much gear for 38’s. Backing her down to 4:10’s would be choice, should put you back in the 19-2000rpm range.

 

[DIESEL JOHN DOE]

You're going to want to go to probably 4.11's to get back to where you were at with the 40's. 2000 rpm at 70mph is the sweet spot for these trucks; too many more rpm's and fuel mileage drops like a rock, too many less and you're waiting on the turbo to spool anytime you get into it. That's how it works with stock engines anyway.

well my f350 i have upgraded to a garret turbo hybrid with wicked wheel it is a 96 not the truck in the profile picture

 

[Goneracin]

I run 37 inch tires with 4.30 gears and it put the truck right back to stock gear ratios. It is so close to stock that I actually disconnected my tru-speed spedometer corrector after the gear swap.

I also use this truck to tow quite a bit, mostly our 8000 lb camper, so towing between 2000 and 2300 rpm is good for me. If you arent towing and want a slightly lower RPM at 80, then a 4.10 will do better for that. Really depends what you use the truck for and how you want to drive it.