GMC Vandura 1994 Extended 2500 gas mileage
 

is there any way to improve the Vandura's mileage

currently it may be getting around or exact 14 Miles to the gallon

That thing is a tank. Unfortunately physics are against you but there is a few things.

First, drive slower and use the brakes less. I typically drive 10 under the speed limit on the highway.

Second get the torque converter locked as early as possible when accelerating. The lockup makes your engine/trans much more efficient.

I'm sure there are other minor things you could do like bumping up tire pressure, removing weight, removing the roof rack/ladder, etc.

what is a torque converter?

does it help the gas mileage a lot going 10 under?
thanks.

It's an automatic

ok is it a manual or automatic?

automatic

It's a physical linkage in your transmission that will connect under the right conditions and allow you to get the same mileage as a manual with the same ratios. Normally in an automatic, the transmission is connected through a fluid (fluid coupling) which is relatively inefficient. The more you have the tcc (torque converting clutch) on, the better your mileage will be, all things being equal.
In terms of speed, probably. If you check out the gas log for my small i4 pickup truck you can see the dip, which was due to cruising ~10+mph faster. If your truck is similar, you may approach 20mpg by cruising at 55mph assuming you get ~15mpg cruising at 65mph. Cars exhibit this too, but the speed where it manifests itself is much higher because they are more aerodynamic relative to their weight.

https://auto.howstuffworks.com/torque-converter.htm

Okay its pretty easy to figure out when its locked or unlocked. I'm pretty sure you have a 4 speed auto. Slowly accelerate and count the shifts. From a standstill you'll be in first. Count the shifts till your in 4th. After a few seconds in 4th going a steady speed the converter will lock. You can differentiate between locked and unlocked by feel. When it locks engine speed will drop slightly. When locked the engine speed will increase steadily as you accel. When unlocked the engine speed will move up and down regardless of speed.