I bought the truck with 3.31 gears ... Can they be changed to either 3.55 or 3.73 ... Would it be a major undertaking?

 

why would you wanna change?? I have 3.31's and they work great especially help with 80+mph fuel mileage. The only negative is if you wanna run oversize tires.

 

More towing, more weight.

 

...seeing that the differentials are the same between the gas and diesel Super Duties, and the .73 is available in a gas truck, it should be easy enough to change them out. You do not post whether you have the Electric Locker option. That may or may not be a deal breaker. I, too, have to ask why? With 18 inch wheels and 3.31 gearing, your truck has the same practical gearing as my very similar truck with 3.55 gears and 20 inch wheels. I regularly tow within 1200 pounds of my GCVWR of 23500 pounds. I have been up and down long 9% grades. I can run 65mph all the way to the top. BTDT. I can not see the additional 1200 pounds I am not towing making much difference up the hill. What are you going to pull? The dually truck used in the "Rumble" to pull a combined load of over 28000 pounds was geared at 3.73. It did the job. Remember the altitude was the only reason the Chevy was faster there. At altitudes below 8000 feet, PUT.com's original shootout has shown the early 390/735 Ford to be stronger up Chevy's own proving grounds 16% hill. It would have been even worse with the 400/800 Ford. Just wondering...not arguing

Hey Cummins, Did you get your truck back?

Regards

 

I don't tow that much, but I am not sure how you are coming up with that. The specs for the F-350 SRW 4x4 conventional towing max are the same for 3.31 and 3.55 at 12,500 and 5th wheel are the same at 16,000. 3.73 is only available on the DRW. I guess you can put what ever you want on the vehicle but now you are outside how Ford designed the vehicle. Must be a reason they don't offer all the ratio's.

 

Final gear ratios are about keeping the engine at or near the rpm that it makes peak torque at cruising speed. Too many people get caught up in old school practices that don't apply to today's diesel pickups. With the transmission gear ratios in these trucks, switching to 3.73 gears is going to be detrimental more often than not unless you tow at max GCWR the overwhelmingly majority of the time.

 

I wanted to "downsize" my ratio from 4.30 go 3.73 and I figured the safest way would be to have Ford do it that way they couldn't deny any warranty for aftermarket parts (ie if they do it they must approve of it). They said they could but it would be in the 3,000 range for both axles. I'm hesitating because I'm doing it for fuel economy, and since I do mainly around town driving with a heavy foot people are saying I probably won't see much of an increase.

 

OK, got it, keep what I have......Thanks.

Primarily curiosity.

 

another thing to keep in mind with 3.31's is that when the load is really heavy when the truck downshifts to 5th its a pretty good place in the RPM band to be at least on my truck at 70mph

 

roadkingfl, I didn't necessarily mean that your desire for different gearing was wrong. As stated in the quote above, your gearing needs to be matched to the speeds you will be towing at and where the different gears match the peak torque and horsepower rpms. It is not difficult to create a spreadsheet to plot this out or you can just use the Gear Calc program to do it for you.

 

Thanks buddy, just thinking it may be more trouble than it's worth. I do have the elock, this may add a level of complexity and expense I don't think I want to deal with, especially since I still want to add the lockpick and the titan tank.

 

The ring and pinion for each axle should be the same regardless if you have a ELD, limited slip or open spool.

 

With the Ford having the torque come on early and stay relatively flat, I wonder why they left the transmission ratio so high.
Compared to the GM at 75 mph the Super Duty is turning about 300 rpm's more.

Using this free program I inputted the ratios from the GMC and Ford when I was researching trucks.

GearCalc

I like just about everything with the truck, but fuel mileage claims I believe have been a little exaggerated. And the ratios might be the reason.

 

Wow, thanks for the specs, very informative.