So I've been thinking I need a lot better fuel mileage and higher top speed than what I've been getting.
I have a 99 F350 dually 6 speed with 4.10 gears, 4" exhaust, intake, aftermarket heavy duty clutch and 6 position chip that I don't do much pulling with anymore.
Talking with Randy's Ring and Pinion my choice's are 3.54 or 3.25 for my truck.
Any thoughts about whether the 3.25's would be too high? I have a ton of power with the current gearing and the chip only set at the lowest setting.
I'd hate to put the 3.54's in and wish I would have gone with the 3.25's. or the other way around and not be able to pull anything (currently only pulling 6000 to 8000 lbs.)
Any thoughts or comparisons? Thanks

 

Anyone out there with 3.54's and a 6 spd? What kind of mileage are you getting on highway?

 

I thought the SD came with 3.73

 

3.73 was an option on the SRW trucks. I can't imagine going with 3.25. I think you'd drop the RPM at highway speeds too low behind the torque curve and end up getting worse mileage.

If you rack up a lot of miles driving above 70 or 75 mph, the 3.54 might work. With 3.73 I turn around 2,000 rpm's in the 65-70mph range.

 

I had a 93 model non turbo with 354's could get low 20's mileage. Even pulled quite a bit. I would get the 354's but would not touch the 325 rear end. Just my opinion. With a 6 speed I think the 354 would be ideal daily driver with limited tow.

 

First of all going faster and higher mileage don't belong in the same sentence. The wind resistance at higher speeds kills your MPG. I think you'd be ok with the 3.25's since you have 6 gears to choose from. Is your truck and early or late 99?

 

The truck was built March of 99, that's a late model isn't it?
I wasn't looking for the best mileage but to improve on what I'm currently getting when I run thru Montana and Wyoming at 80 to 85 i'm only getting 12 to 13 MPG now.

 

Hmmm yeah I'd do the gears and maybe a T500 HPOP if you have low ICP pressure. That T500 alone gave me 1 MPG, but my ICP was really bad/low. Make sure you have a lot of air in the tires as well.

 

Correction, the truck was built in Feb of 99'.
I ordered the truck with the 4.10's cause I was doing quite a bit of heavy towing over the mountain passes back then. I'm out of the horse business now and the heaviest thing I am pulling now is a 28' boat but I don't think it weighs anymore than 8000 including the trailer. Also I'm not running it over the passes, mostly on the flats to the bay.

 

Fat Diesel, which gears were you referring to 3.25 or 3.54? Like i said before loads of power now with current gearing and the chip.
I'm running max tire psi in the fronts and minimum pressure in the rear duals, it's the only way I can get any kind of tire wear without a load.

 

I was thinking the 3.25's. With the 6 speed and the granny low you should still be ok towing, just not in overdrive. How many miles a week or month or year are you driving the truck now?

 

That's kinda what I was thinking/hoping with the 3.25's.
I'm only using it about 10-15k per year now maybe 10% of the time i'm towing but only short runs.

 

Dave Whitmer drives a 7.3 2WD with a manual transmission with a 3.08 ratio differential and gets great mpg. See:

Improving MPG - PowerStrokeNation : Ford Powerstroke Diesel Forum

He discusses other stuff which doesn't work in the following sticky:

http://powerstrokenation.com/forums/...9746&highlight=

Finally, he addresses the whole tire size deal in the following sticky:

http://powerstrokenation.com/forums/...9458&highlight=

I wish he would become our mileage moderator over here too.

When I first bought my van driving 45-55 on the flats down I-5 from Sacramento to L.A. I got 21.96 mpg hand calculated with the original 2WD 3.55 differential. The 3.55 differential had enough power when the van was 2WD. The 4x4 converter convinced me to go to 3.37s when he converted it to 4WD. Now I get 12-13 mpg regardless of how slow/fast I drive. In effect, I have lost my overdrive. I plan eventually to install a ZF6-speed with 3.25 or 3.08 differentials. Between the two diffs you are considering I, personally, would go with 3.25. But pm Dave first.

 

With the setup I have granted I have an auto. It puts me around a 3:12 year if my calculations are correct. The truck still pulls fine and gets out of its own way with ease (I wouldn't pull anything that's really heavy though). My mileage dropped from 16 on its best day to 13-14mpg constantly. Egts can get too high too fast. I'm pulling between 650-750 degrees empty at 55 mph annd 700 (give or take a little) at 70 on the highway all on flat road.
I'm debating doing the complete opposite of what your wanting to do because I don't dd my truck or drive for mileage. I'm looking into 4:56s or 4:88s though I know 4:88 would be a lot more than I would ever need I know it would pull like a train and get horrible mileage but I've come to terms with that and am using it as a play toy now more than anything.
If your truck is 4 wd your looking at over 2 grand for a gear swap so you do the math.

 

It is a 4wd, I was quoted $1800 front and rear $2400 with bearings out the door.
It's a want not a need.