have a 2001 f350 7.3 diesel 4 x 4 with 8 inch lift. have 35 and will be going to 37's. shoulkd i upgrade gears from stock 373?? if so what gears should i go with, 410, 456???? plz help?? use truck once in a while for towing full size f150 or mustang. thanks. rob.

 

4.56 or 4.88 depending on how much and how often you tow.

 

If you have 3.73 now the 4.30 will put you just below stock and 4.56 will put you just above stock.I would think 4.88 would rev a little high for the 7.3 on the hwy.

 

I love my 4.56 with my 35's, they made a good difference over the 3.73s. For your 37's 4.56 will work, but I'd also consider 4.88.

http://www.et-studios.com/motorsports/gears/gears.html

Your 4th gear is 0.71:1.
With 4.56's and 37, at 2000 rpm you will be doing 68mph
with 4.88's and 37, at 2000 rpm you will be doing 64mph

I like to use 2000rpm and my cruise speed to pick my ratio, 60mph is a pretty good cruise and offers decent performance.

 

This is what DCspecial wrote up on another site... I stuck with my stock gears for my 37's... love the mileage.. but if you tow more I would go with 4.30's... Unless you plan on going bigger someday...

Here's a quick example of rpms with given gear ratios at 70 mph. All rpms listed are for a AOD trans with a .70 OD ratio....so they aren't dead on, but they are close (the 4R100 has a .71 or .72 OD ratio I think??). So, while the rpms aren't 100% on the money it will give you an idea of how each different gear ratio effects engine rpm at 70mph.

tire..............gear...................rpm
31.6"...........3.73....................1943 (stock truck)
31.6............4.10....................2136
31.6............4.30....................2240
34.7............3.73....................1770
34.7............4.10....................1945
34.7............4.30....................2040
34.7............4.56....................2164
36.7............3.73....................1673
36.7............4.10....................1839
36.7............4.30....................1929
36.7............4.56....................2046
36.7............4.88....................2189
38...............3.73....................1616
38...............4.30....................1863
38...............4.56....................1976
38...............4.88....................2114
38...............5.13....................2223
40...............4.10....................1688
40...............4.30....................1770
40...............4.56....................1877
40...............4.88....................2009
40...............5.13....................2112
42...............4.56....................1788
42...............4.88....................1913
42...............5.13....................2011
42...............5.38....................2109
44...............4.56....................1706
44...............4.88....................1826
44...............5.13....................1920
44...............5.38....................2013
46...............4.56....................1632
46...............4.88....................1747
46...............5.13....................1836
46...............5.38....................1926

 

looks like my truck should get better mileage when I go up to 37's with my current 4.30's... with a diesel, I wouldn't go over 4.56's and 37's... they have plenty of torque as it is... having 4.88's and 37's (on the diesel) would be like if I wanted to run 5.13's with 37's on my 6.8L... my .02

Joey

 

Diesel makes a big difference, since they have 2000rpms less available to them and a turbo charger. So disreguard my info.

 

I'm in the same boat myself as I just lifted my truck 6 inches with 35s. The stock gears are ok, but they're killing my mileage and some of my power. I spoke to a number of shops about some new gears and I'm not sure what to think. My truck for whatever reason came with a 3.73 open and I'm wanting to go with either a 4.10 or 4.30 limited slip, but I'm not sure what would work better seeing as I occasionally tow and do most of my driving on the freeway. I'm also not sure as to what brand I should look into. What and who do you guys recommend?

 

I have the FX4 package which came with the 3.73 limited slip differential and let me tell you, it would be more appropriately named "unlimited slip". I went up one size on the tires, from 265/75 R16 to 285/75 R16. That's a 33" tire, or close to it. I run right at 2000 rpm going 70 MPH, and my speedo tracks dead on with the speed on my GPS. On my recent trip, distance traveled was maybe 5 miles off over a 900 mile trip, GPS to odometer. I am considering going up to 35" tires for the next set, maybe two or three years out depending on how much mileage I put on the truck instead of the bike. I may go with 4.10 gears if I get a TT or 5er, but if not then I think I'll stick with the 3.73 gears. I have been thinking about replacing the junk "limited" slip rear differential with a Detroit Locker, Tru-Track, or maybe ARB air locker even if I don't change gearing, and maybe even a Tru-Track for the front. I have a 8' plow so it would be for traction when plowing, not for heavy off road use.

 

With 37s, I'd go with a 4.30 gear.


Personally, I wouldn't regear a diesel with a 35" tire unless you tow heavy.....I got my best mpg on my 01 with 35s and 3.73s.
If you tow with a 35 and feel you need to regear, 4.10s.