So I just bought a 1994 F-350 XLT, crew cab, long box truck with 113k miles for $2k. Has 351 engine with 4.10 gears, manual 5 speed.

When I drove the truck home I got ~10 mpg, so I would like to put 3.55 gears in it. I have looked on this forum for this same swap out and people have gotten up to 19 mpg with this truck. Not worried about towing capabilities.

What I would like help with:
1. Are the front and rear pumpkins the same size and gear ratio? It is 4x4.
2. Where can I find the correct gears to put in the front and back axles of the truck? Websites or local stores that I could check out. Or even used ones from people on this forum.

 

On edit, I see you added the powertain info. Maybe 10 seems a little low for that combo. Have you done the usual "tune up" checks and fixes?

Gear sets are about $200 each. Your front and rear diffs will probably use a different gear set.

Used gears? Forget it. How would you adjust them to pattern correctly? They would soon be howlers in most cases. The gears are the cheap part anyway.

Expect another $500 to $1000 for labor and other parts. (bearings and gaskets. Don't use the old bearings again)

19 out of an F350? On a flat highway with no wind perhaps.

 

What gears are in it now? 19 MPG with a F-350? Going from 10-19 mpg just by changing gears? Tell them people they are full of it. SORRY.

 

At best I used to get ~12 MPGs on the highway running slightly under 65 MPH with my old 1992 F350 4x4 single cab w/5.8L, E4OD, 3.55 gears and stock height tires. Wherever you thought you saw 19 MPG you must have been dreaming. Perhaps you spied a thread of someone with a diesel? Even then, I think that is a stretch.

Swapping gears down from 4.10 to 3.55 is not going to gain you anything. My truck was a slightly smaller version of yours and I considered it mildly adequate to get the job done. I was seriously contemplating going to 4.56s just to gain some more torque multiplication. To answer one of your questions: the front and rear axles are different you need to two different part number gear sets if you want to proceed with a gear swap. The money to do such a swap is going to take a LONG time to recoup and you will never see anywhere near 19 MPGs with a gasser F250/350 IMHO.

Start with the basics, i.e. a complete tune-up with goof quality filters, cap/rotor, plugs and wires. Search for the "Sixlitre tuneup". Basically a wider plug gap, hotter coil, and a base timing bump. Check the brakes to make sure none of them are dragging. Keep the tires properly inflated. Drive like there is an egg under the throttle. Check for codes even if you do not have a Check Engine Light (CEL) on.

Stay away from cold air intakes, throttle-body spacers and "performance chips". Complete waste of time.

If you really want 19 MPG and above sell the truck then look at buying a Ford Focus or similar gas sipper economy car.

 

Are the tires stock size?

At what indicated speed? Like the others have said you're not gonna get anywhere close to 19mpg from a crew cab 4x4, but it should do a little better if it's in top running order. Check for codes, do a tunup, if the tires are oversized recalibrate the speedo, if the tires are stock size(see the calibration decal on the drivers door pillar) and after you get done fixing any engine issues you might consider installing oversized tires to decrease the final drive ratio a bit. Also check that you don't have the fuel transfer problem if this is a dual tank truck, there is an issue where the check valves that are supposed to prevent crossflow between tanks get gummed up with dirt or varnish from the fuel and stick open, and that can put a big dent in your fuel milage

 

19 MPG? I doubt it but you should be able to get into low to mid teens with the swap to 3.55s, all the tuneup stuff, and (most importantly) what you do with your right foot. Especially since you have a 5 SPEED! If you need that higher (numerically) gearing for doing some stump pulling, etc. you've got the low range of your four wheel drive transfer case. I was able to gain a DOCUMENTED average 1 MPG increase when I did this swap.

Gear swap? I wouldn't (unless you know it is toast anyway) - since if you are patient and shop the self service yards you should be able to come up with (at least the rear - a whole 10.25 axle) for under $200. Be a bit discriminating and you may even score a limited slip unit. I find them regularly here in the midwest (I failed to note your locale). Your front (Dana 60) may be another story, but try to work a trade. This shouldn't be too hard since most guys want to go the other way; from 3.55 to 4.10. You may even be able to score both in a trade.

FYI: Assuming the stock axles are still in your truck the door decal axle code will show "35" if open differential, "C5" if limited slip. You are looking for a unit coded "39" [open] or "C9" [limited slip].

To build on the "right foot" comment above, if you have never done so get a vacuum gauge and install it in your truck. It will teach you so much about driving for best MPGs! Once you've done this for a time you can improve your fuel efficiency in any vehicle you drive.

How about the exhaust? Does it still have that huge and restrictive (even more so as time goes on) catalytic converter and less than 3" tailpipe? How far do you want to go? Long tube headers? Intake/cam/head swap?

With your four wheel drive and its higher stance you inherently increase your drag coefficient - something you really cannot do a whole lot about. In any case, welcome to the Crew Cab Club!

dn.

 

Guys what about removing some of the smog controls? Would this allow me to get better mpg? If so, which controls would I remove?

 

Sorry, truth might hurt, but you will not see over 12mpg in that thing. Heck the 2wd F150's with that driveline have trouble getting 13 and 14 and they're half the weight of your truck!



And screwing around with emissions controls on EFI's isn't the best idea. It can actually make it run rich and get really poor mpgs.

I know you can remove the A.I.R pump though as it's fairly useless and causes drag on the engine is the bearings are getting old.


You will need a shorter serp. belt though.

 

Yup, sorry bud but 19MPG ain't happening in that truck. Get her running good as mentioned above and be glad that you get slightly better mileage than the 460 guys.

As for what an F-150 can do, I've pushed 18MPG in my SuperCab shortbed 351 auto 4x4, but I was on open highway doing 60 behind a semi for several miles. Usually it's around 14-15.

 

Hi,
My 93 f250 hd ,5.8,5spd,4x4 sc lb with 245/75r16 and 3.54s gets about 11-12 mpg , it is geared too tall for most use. iirc 70 mph is just over 2000 rpm . Which is below optimum engine speed for a gasoline engine . I won't change either tire or gear as benefit would be too small. My plan is that by adjusting fuel/air leaner, I might get to 15mpg . Oh and with minimum viscosity gear lubes and any other of the eco-modder tricks. And keep looking for Cummins,other diesel engine.