I Have An 88' F-250 7.3l Diesel It Is All Orig. And Only Has 39k On It.i Want To Swap The 4.10 Rearend With A 3.55 Rearend So I Can Get Better Mileage. I Dont Do Alot Of Towing And When I Do Tow, My 5th Wheel Only Weighs 6500lbs.does Anyone Know How Much This Will Help My Mileage And How Much It Will Take Away From My Towing Capablities. I Thought About Swaping My C-6 To A E40d But I Have Heard That Would Be A Bad Move?. Thanks Derek

 

You are right on staying away from swapping the C6 to an E4OD. Very expensive and you are potentially upgrading to a less reliable transmission unless you spent even more money to get an aftermarket upgraded E4OD.

I have been thinking about swapping my 4:10 in my 91 F350 to 3:55. The truck is a 4x4 so the swap is not a cheap one to do both axles but probably still the greatest bang for my buck when you look the the true alternative which would be an aftermarket OD unit such as US Gear Splitter or a Gear Vendor. My 91 gets 14 mpg city, 15 mpg highway.

My 93 with the turbo, ZF5 and 3:55's will do 18 mpg city and 20 mpg highway.

I figure swapping your truck to 3:55 should give you anywhere from 1 to 3 mpg better than what you do now. 3 mpg would probably on the optimistic side of things while 1 mpg should be easily achievable.

The impact on towing with the 3:55 should not be that bad given that you tow fairly lightly.

 

HAMBERGER, THANKS FOR THE INPUT! WHAT ARE YOUR THOUGHTS ON MODIFYING THE AIR INTAKE AND PUTTING A 4" EXHUAST, I HAVE READ THAT THE 2 BIGGEST THINGS THAT HOLD THE 7.3L BACK ARE THE INTAKE OF AIR AND THE OUTPUT OF AIR. IM WONDERING IF I DID ALL THREE (rearend,intake,and exhuast) HOW MUCH WOULD THAT GET ME? 5 TO 7 MORE MPG??

THANKS AGIAN,
DEREK

 

I have done the air intake on both my 91 and 93. I did not go as far as D.S. and make a scoop. I just took a stiff 4" marine exhaust hose, reused the old plastic flex piece and ended the hose on the drivers side of the radiator (not perfect but I figure it give me 90% of the benefit for 20% of the work. It is not as good as a ram air but getting rid of the Ford plastic hood scoop helps a lot. The other thing I did on both trucks is I went to K&N filters. The difference really shows. I used to show a vacuum on my Turbo filter inlet housing gauge all the time with the Ford paper filter after I spun the turbo up to 7 psig. With the above modes there now is virtually no pressure drop on the inlet to the turbo.

I have considered the exhaust mods (say a 3" or 4") many times but I have resisted due to cost. The ATS 3" downpipe and exhaust for the 93' turbo is a real tight installation and material alone from ATS are not cheap. It would take a lot of fuel savings to pay this back.

The last owner of my 91 put in a brand new 2 1/4" exhaust before I picked up the truck so I am having a hard time ripping it out and replacing it.

The larger exhaust will give you fuel savings more with Highway driving than city driving as the restriction really becomes apparent under load hence my below par increase in milage between city/hwy (18/20) on my 93'.

With the inlet/exhaust mods plus the 3:55 gearing I think a 5 mpg increase is still optimistic in my mind. 2-4 mpg is probably more in the ball park.
The mods you mentioned are about the cheapest things you can do to increase milage short of spending several thousand $ for an additional overdrive unit.

 

Thanks For The Info.


Derek, Ks