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I have a 99 F350 CC Dually V-10 w/ 4.30's in it right now. Over the winter I want to change the gear set, It is only RWD and I only use it to pull my Bumper tow 30ft Jayco Jayflight (9000lbs) I already have a 5 Star tune and will be doing banks headers this winter as well. Other than tune truck is completely stock. Taking a long camping trip come April and am looking for a little extra power. I would like to be able to use the OD more instead of dropping into 3 just as soon as you start up a hill. Only seeing 8 - 8.2 mpg anyways so what are your thoughts.

Thanks

Angelo

 

Don't know if this help but here's a couple of pictures



Angelo

 

My 01 F350/V-10/CC/LB 4x4/4:30 gears.... pulling my 36' 5th wheel gets right about 7/5-8.5 mpg. I have the 5Star tune also using 89 tow tune and cannot hold overdrive much either and always shift down to 3rd.

 

I'm not sure what size your tires are but my Excursion was running 32" Continentals when I regeared from 3.73 to 4.88s. At that time we were towing a 9,500lb toyhauler and with the stock gears it was seeing 6 to 7 MPG running up/down the East coast on I-95. With that setup it would downshift out of OD if we ran over a shadow.
With the change to the deep 4.88 gears and all other speeds and conditions the same we saw 9 MPG on the same route with the same trailer. And it didn't downshift on the highway a single time between Philly and Savannah.
With those gears and 32" tires it was spinning 2430 RPMs at 65 MPH in OD. And my unloaded mileage dropped from 14-ish to 12-ish with the new gears.

 

Tires are 265/75/16

 

So about 31.6" tall, you will be amazed at how much better the truck tows with the new 4.88 gears.

 

Aside from the ring & pinion what else is needed. Mine is just RWD. What about speedo?

 

I think buying a Master Rebuild Kit which includes new bearings and crush sleeve along with the new gears is a wise choice. Having a shop that has a good reputation and that does lots of these diff rebuilds is the best bet too, it can be done by the DIY owner but there is some opportunities to miss the correct setup.
Lucky you, your RWD truck will be half the cost of doing a 4X4!

The speedo is driven by the tone ring on the ring gear, so it actually reading tire/axle shaft revolutions per mile. So a gear change will not affect the speedo, my speedo was reading the same with my 4.88s as it did with the 3.73s. A tire size change will affect the speedo because the taller or shorter than stock tire will be covering more or less ground per revolution than the stock sized tire. My factory 265/75R16 tires were 31.6" tall and now with my 35.28" tall Nittos my speedometer/odometer reads about 12% low.

 

Since you already plan on Header's I would try them prior to a gear change and see if your truck holds the gears better on the hills.

 

Yes, I am going to do headers as well.... Which ones? Thorleys or Banks?

 

I went with Bank's and are very happy with the quality

 

I run the Banks headers also, very high quality stuff!

 

I have the Thorley's sitting in a box. Look like nice stuff. I ordered them through 5star tuning back in July. Couldn't take the truck down during summer to install and to cold now. However specify when ordering if you have a EGR on the drivers exhaust manifold otherwise you'll be trying to figure out how to bypass.

Fortunately 5star is willing to write a tune for emissions purposes, I will have to figure out a way to delete the EGR mess though.

 

I was wrong on he tire size.. Went and look at it yesterday and they are LT215/85R16 Firestone TransForce... Will that matter.

Thanks

Angelo

 

Yes.
That tire size is shorter than what is usually on these trucks. They are usually equipped with 235/85/16 or 265/75/16, which are very close in diameter. The 215 is smaller diameter so the tire is spinning more revolutions per mile and making you "think" you are traveling farther than you are. So any mpg calculations are going to be skewed because of the different tire size.
The speedometer will also be off by about 3 mph too fast @ 60 mph. So if you are keeping up with other traffic that IS traveling at the correct speed limit, your truck will be turning more RPMs than stock due to the change in effective final drive ratio which will also reduce fuel economy.
Your truck should have a vehicle certification label on the driver side door jamb that will have the original tire size on it.