In a way it is because the engine can breath but at the same time it still kinda hurts it. One of those grey areas where the only way I'd know for sure one way or another would be to sit there and test it myself from one to another. But yeah a good 2.5 or 3 inch exhaust is a good size for a truck like yours. As for miledge well the old six liter tune up is the best route to help with that. Fresh cap wires plugs heck even a coil while you're at it rotor and seeing if you can up the ignition timming from 10 degrees to 12. You get a little extra zip doing that and slightly better fuel mileage. A good oil change with whatever oil you like best and filter and boom that's about it for the six liter tune up.Just remember to keep your foot out of it with your new found extra ponys.

 

Let's step back a bit.....Is the Check Engine Light on? Even if it is not you may have stored codes. So it is highly recommended to run the Key On Engine Off tests and stored code display(Continuous Memory). If everything passes you will get Code 111 (System Pass) for the KOEO tests and the same Code 111 if there are no stored codes.

When was the last time the cap, rotor, plugs, wires and fuel filter were changed?

Your truck has two converters from the factory, did you punch both out? It would be better advised to remove them completely instead of leaving them in place. The gutted converters act like an expansion chamber and can inhibit exhaust flow at different operating conditions. Even better is to replace the two converter setup with a single high flow converter. Some of us like breathing clean air. For that matter long tube headers with a proper sized exhaust pipe would be a big step in the right direction.

14 MPG is not bad, but it could be better. A base timing bump is HIGHLY suggested. As I stated before your truck already has a decent roller cam, suggest 1.7 ratio roller rockers for some extra lift.

 

The 302 doesn't make a lot of low end torque, so anything over 31 inch tires doesn't help matters either.

That diesel exhaust is definitely too large. Stay with a 3 inch single or even a 2.5 inch single.

The KN FIPK "hot air intake" is doing you no favors either. The only reason you feel it helped is that you can hear it. Its the placebo effect. Just like people think the loudest exhaust is the best. The stock intake is better.

I don't see any reason to dyno it. As long as it runs good and doesn't smoke and isn't really down on power, its probably in decent condition.

If your not attached to the lift, I say ditch it and put the truck back to stock height.

 

steps to get better MPG out of these trucks

Step 1: get slightly smaller tyres
Step 2: get better gearing ratios if needed
Step 3: take your foot off the gas and go 65
Step 4: install cruise


if that doesn't work try these steps

Step 1: get a beater car that sips gas
Step 2: don't expect the truck to be a high MPG vehicle (its a truck)
Step 3: be happy your getting decent mpg
Step 4: get rid of the truck


Moral of this list?

if you need an econo car get a kia or something small like that, if you need a truck to work use the truck, these ain't going to be fuel efficient trucks by there inherent nature!!

keep in mind the more HP/faster you want to go the more energy you need (gas)...

 

If you change tire size you change the speedometer and odometer calibration, bigger tires will make it read slower and give a lower than actual mpg by the odometer reading.
I'm very skeptical about whether having too large of an exhaust system really affects power or gas mileage all that much. I have gone from stock to custom slightly bigger exhaust or no exhaust and have friends that have gone from open headers to stock exhaust and vice versa with almost no power or mpg change. I think it's more of A mental thing, louder exhaust does two things to people #1 it sounds good and they drive harder = less mpg #2 it sounds like it's working overly hard and think it has less power.
I'm most likely going to be told I'm wrong on the on the exhaust stuff, very opinionated issue but have seen Chevey 350's Ford 460's, 351's, 5.4's 2.9's and V10's 2/3V run virtually the same with many exhaust changes from a GOOD stock exhaust.

 

Pretty decent guide to improve everything:

FOUR WHEELER Magazine's Project M.P.G.

 

Hmm okay, so exhaust is to big. I think I only have one punched out cat and that muffler. I'll look into a different exhaust. Also the k&n is less helpful? Or just the same? And then I will go down to 31's or 30's next And do less agressive tires too. and I'll get a timing light and bump it just the slightest bit, what info do I need, is it clockwise or counter? And how do I know how far I should go? Thanks for all your guys help

 

Read the above guide.
Pretty much every concept from the 7.5L applies to the 5.0/5.8L engines.

 

Yes very helpful! I'm reading it now and appreciate the info and numbers

 

It is an incredible resource - I've read through it numerous times.
The only upgrade that's a little far-fetched is the GV Over/UnderDrive unit.

 

Okay so it is fair to assume that my cold air intake from K&N did help? good to know. I wanna know more about that hypertech module

 

Are you talking about the FIPK? The filter where you delete the factory airbox?
Or the K&N that goes into the factory airbox?
Results showed the K&N filter that goes in the factory airbox, with thermal barrier tape, was the best out of the 4 options.

FOUR WHEELER Magazine's Project M.P.G.

I dont think HyperTech makes chips for these trucks anymore, may contact them for more info.

 

Yes the FIKP. I know the one with the Factory air box did the best but the other still did good, I'm just glad it wasn't a complete waste of money.

 

I won't use KN of either style. Its a lousy filter and a gimmick. The replacement doesn't really fit the box all that great either.

 

he below links are for the six litre tuneup and a ultimate gas mileage thread. If you haven't given the truck a good tune up that would be the place to start.

ignition upgrade and timing bump (no 56K) - FSB Forums

The ULTIMATE better MPG thread! - FSB Forums

As far as your plan for potentially upgrading to a 1992 diesel, those are naturally aspirated and usually only get around 15-16 mpg, fuel is $1.00 more a gallon at the moment, and general maintenance costs more (i.e. takes gallons of oil vs quarts). It would most likely cost you more money to run the diesel than your current truck.