i used to have a 65 inline 6 300 with a three on the tree.
it got about 20 mpg . i just recently bought a 93 inline 6 300 with a manual 5 speed figureing it would at least get as good mpg but it is not the case . ive read some of the articles here suggestion to remove the cat ( live in fl and they dont check smog ) some say that will help mpg others say that will hurt mpg a little. it does make sense that the cat will restrick exhaust flow. what you say
and why

 

That 93 is a different beast than what you used to have. It is choked by emmisions add-ons, which is why Ford stopped putting it in vehicles.

You're not going to reach the MPG you used to get out of the old one.

 

slice and dice the kitty! just finished the front one yesterday. if yours is like my 302 and you have 2 cats, we found it easiest to drop the crossmember to get it out to gut it. unbolted at the manifolds, sawzall'd between the 2 cats, hollowed 'em with some rebar and a nice hammer, weld back together, and enjoy polluting the Florida air with me!

 

Kill the cat and never look back

 

Unless the cat is plugged, leave it alone. It's designed to run with the cat in place. Unless you have major engine mods, the cat isn't making enough restriction to rob you of any power or MPG, so you won't gain anything by removing it.

Also, if I remember correctly, the cat on these is a dual in/single out that attaches directly to the manifolds. That means you can't just cut the cat out and replace it with a piece of pipe. There's going to be custom fabrication involved.

What are you getting for MPG now?

 

this is true about 2 in/1 out of the first cat, but if you're only planning to hollow it out and not replace that section of exhaust, the only fabrication will involve a sawzall, hammer, rebar, a welder or a coupler with clamps. i haven't noticed any change in MPG or any increase with my highly tuner butt-dyno, but what's the hurt with a more free flowing system? (note: i'm NOT saying that leaving the cats alone restricts unless they're clogged!)

when i did both cats, neither of them were clogged. the one thing i noticed was a change in the exhaust note, but that's about all. looking back, if i were compelled, i guess i could have just removed them all together and sold them. ah, hindsight?!

 

not sure what kinda mpg your gettin but my 300 is getting 15ish

 

it semms that the cat does nothing to improve or detract from mpg so for now the cat will stay . however it seems there is there is a big international plot to make me send more money over seas to people that hate us so they can buy more ak,s. how else can one explain my 65 300 6 3 on tree getting better mpg computer controled fuel injected 5 speed manual with overdrive and same motor. of course it could be that bussiness and govt are complete a holes. so i shall set it upon my self to get this thing to better than 20 mpg for the good of the America that i love ( and conserve some green)

 

Keep in mind that those old 300s had a tiny little 1 barrel carb. The EFI 300 puts a whole lot more fuel/air mixture into the engine and produces a lot more power.

 

If we ignore the engine differences for a moment.. which are significant and can impact fuel milage, the rest of the powertrain also has a big impact on milage. What axle gearing is in the '93? Florida is mostly flat(I think) so vehicles can get away with higher gearing but if it's too high you could be lugging the motor in OD and with EFI that can produce worse milage than if you reved it a little more and ran at a lighter throttle... i.e used 4th instead of 5th. 20mpg is possible from an EFI 4.9 there are guys here getting that, but only with certain powertrain combos.

 

i have considered changing out the rear end however there is the the fact that you really dont know what rear end gear gives the bestresults unless you can expeirment with a few of them, not practical.
so im tossing around getting the software and hardware to change the settings in the computer and thereby have greater control with the least amount of work. and i should be able to get rid of the egr totally and control the o2 senser and injectors at the same time. it might be crazy but it makes sense to me what you say

 

Not so, optimum fuel milage will be attained with something like 1800rpm at highway cruising speed, so we just need a few pieces of information about your truck. What is the rear axle ratio(see the axle code on the drivers door decal). What sizs tires are on the truck? How fast do you typically drive?


Nice idea but there isn't much to be gained there. EGR actually improves milage, the O2 isn't controlllable and neither are the injectors directly, and besides the stock calibrations will deliver a pretty lean burn 14.5-15:1 when presented with the right conditions, and that's what you will accomplish with appropriate gearing.

 

my tires are 30 9.5 15 my door tag is gone but the tag on the rear end is 5838b 355 88 3f14 which i couldnt find anything on alldata that had numbers like that i guess its a 3.55 8.8
alldata says the pcm is set to a varity of things one of which is the rear axel ratio so if i change that would the pcm recognise the axel and adjust for that or does it just try to get the best fuel ratio
iget about 15 mpg

 

Here is my running average:


I have 1 or 2 fill ups to add so its slightly behind. The fuel I got the other day is non-ethanol & plan to run it for a while to see if there is a difference. Another thing I HAVE to deal with is the OEM plastic vacuum lines. Pretty sure they are cracked some cause I still have a slight stumble right off idle.

E4oD, 8.8" 3.08, 235/75r15

 

Yes that's right. Those tires are larger than stock(28.5" diameter) and that affects your fuel milage in at least two ways.. both related to the speedo. First of all it makes the speedo read low so you're actually pushing the truck faster than you think, and it also makes the ODO read low so you're actually travelling further than it indicates, and together these things could account for a 1-2mpg loss right there.


The PCM may be calibrated slightly differently for various axle ratios but it's not a very significant difference.. not something you could notice if you did change the gears.

So that's two of the three things covered, now we just need to know how fast you typically drive on the highway.