So basically, nothing.

[Corts60]

I notice nobody mentioning drive train. Friction is powerloss. So is is slop in gears. When were all wheel bearings last replaced? How many miles on the trans? What are the shift points set at on the trans? Last time you inspected back lash on rear end? Last time you inspected u joints? Do you have a carrier bearing? All these components will eat up up your power if they are not functioning as new. I'll bet if half the guys that think they need an engine rebuild were to replace those components first, along with checking proper tire inflation, alignment, etc., the improvement in their fuel mileage would probably make them say never mind on the rebuild.
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[Rubble]

Corts is correct,it's the little things like he mentioned adds up.Stuff like skinny tires,correct air pressure makes a difference too.You got to remember too,the aerodynamics of these old trucks are like pushing a barn door down the road.It wasn't as big an issue too,because fuel prices were a lot less then too...
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[RPOZ11]

My approach is with my 75 SWB build.
I went bigger cubic inches, 9:5.1, TH 400, and 2:56 gears.
I went with not spinning the motor so high into the rpm ranges and will rely on the torque it provides.

I am "hoping" for 20 mpg, but wont know for a few more weeks.

[Skunksmash]

Originally Posted by Corts60

I notice nobody mentioning drive train. Friction is powerloss. So is is slop in gears. When were all wheel bearings last replaced? How many miles on the trans? What are the shift points set at on the trans? Last time you inspected back lash on rear end? Last time you inspected u joints? Do you have a carrier bearing? All these components will eat up up your power if they are not functioning as new. I'll bet if half the guys that think they need an engine rebuild were to replace those components first, along with checking proper tire inflation, alignment, etc., the improvement in their fuel mileage would probably make them say never mind on the rebuild.
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Now we're getting somewhere. Those are things that I can check and replace without a boatload of work and/or money. Rebuilding the engine is a lot of work and costs a lot more money. And from what I can tell, you rarely gain MPG from it.

[Rufton]

I rebuilt rear on 66 truck w/ about 190,000 miles.
I log all mpg numbers.
All new rear axle/diff bearings.
Built to Spicer spec; new shims etc.
Mpg improvement was nada; YMMV.

[Skunksmash]

Maybe the loss is in the old transmissions then.

[Rufton]

The mpg loss may be any combo of everything everybody mentioned (that helps). My money is on AFR, engine design and gear ratio. Hope you figure it out

[dubds10]

I'd say you could rebuild your truck from scratch and you may see a 2MPG improvement lol It's one of those things I've noticed on every truck I've had from the '50s up to the 2000s. The mileage they go when worn and beat seemed to be the same they got when brand new and nothing really changed it. From what I can tell now-a-days, the manufacturers are over-estimating mileage and of course they are putting thinner oil in some engines which require thicker oil (Jeep) and running in a controlled environment to get the best results.

I get around 5mpg on a good day with my dually right now. I don't drive much since I work from home and have work trucks for when I do go to work. Otherwise my dually is used for daily driving when I leave the house. $100 a week and I get MAX 200kms to a tank. That's 125miles to 1 tank of gas which costs me $100 to fill at $1.45/L which I think is around $5.50-6/G for you Yankees If I go across the boarder I only need to spend $80 to fill a tank, but I've been too lazy to do it lately.

Also have a '92 Honda Civic for when I'm feeling really cheap and don't want to waste the fuel in my dually haha. It gets much better mileage. Can pull almost 600kms to a tank if driving light on the highway and it costs $60 fill.

Fiancee has a 2010 Jeep Wrangler JK which is her daily driver lifted and on 35s. It's $70 a tank and gets 450/kms roughly. OK mileage when it's on the highway, but she fills up every 4 days. She wont drive the Honda to work, she's set on driving HER JEEP even if it drink fuels and wastes $$$.

Bad gas mileage is something we live with if we want to drive these things daily or we get Econo-boxes to make up for it. But they aren't "Cool" to drive and lots of people can't afford the extra insurance and car payments if buying new. For what's it worth, I can buy a 2000 Honda Civic for say $1000 and drive that during the week for work and that car would've paid for itself in the first two months compared to driving my truck everyday. Comes down to your situation and what you can or can't afford. My old '72 Blazer was my daily driver and I never kept track of mileage. Loved that truck too much to really give a damn about how much $ I spent in gas. Pretty sure it got between 8-10mpg with a SBC 350/TH350/3.73 combo.

Only trucks I see with really good mileage for long trips are diesels or flex fuel trucks. My work truck 2010 Ford F150 King Ranch with flex fuel is $130/tank and goes 700kms to a tank. My dad's 2002 Dodge diesel 3/4T coming in at 7000lbs daily driver goes the same distance as the Ford, but costs less to fill so better mileage when empty on long trips. Short trips ruin the mileage of course.

[Skunksmash]

So to sum up:

1. The automakers were lying.

2. The government was lying.

3. All parts that are a factor have aged, thus decreasing MPG.

4. The aerodynamics of these trucks sucked anyway.

5. Most drivers don't know how to drive to get the best MPG.

6. There are so many things that its near impossible to tell what is greatest affecting your MPG.

[Rufton]

My most successful mpg changes to date:
On one car I gained approx 1 to 2 mpg hwy after swapping 3.0 to 2.79 rear.
On truck I gained approx 1 to 2 mpg hwy after installing smaller carb jets.

[bohmankid]

Best gas mileage boost I got was ls swap.
Old TBI was getting 7 mpg in my lifted 3/4 ton.
Swapped to 5.3 vortec (and KEPT the th400), now getting 14 mpg.

[Rufton]

Originally Posted by bohmankid

Best gas mileage boost I got was ls swap.
Old TBI was getting 7 mpg in my lifted 3/4 ton.
Swapped to 5.3 vortec (and KEPT the th400), now getting 14 mpg.

Not bad

[gchemist]

ZZ4 355 HP 405 ft/lbs toque
13-15 MPG city if I keep my foot out of it
18.5 MPG at 70 MPH. Better if I slow down to 65.
Torque is king to better milage!!

[lap15]

Realistic mileage for a tuned up, well running, stock 2wd square is about 15mpg (if you drive carefully around the speed limit without a load, maybe 16). Don't forget one thing when calculating mileage- I have never met a fuel gauge in any square that was accurate! It could show 3/4 of a tank, you take a sharp turn and now you're down to 1/2 a tank. Squares also tend to show "E" when there is still plenty of fuel left in the tank. If you really must know the exact mpg- then the only way to be sure is to fill the tank and run it until it's dry and the truck stops running. Naturally you'll want to keep a jug of gas in the bed so you can get home!