transmission upgrade

I have an 85 C20 with a 6.2L diesel and a 4speed granny tranny and 4.10 rearend. The engine is revving pretty good at 65mph. I was thinking about putting in a later model transmission, to try to slow engine speed down at highway speeds, to get better mpg. What would be a good tranny swap for this?
Clark

I have an 85 C20 with a 6.2L diesel and a 4speed granny tranny and 4.10 rearend. The engine is revving pretty good at 65mph. I was thinking about putting in a later model transmission, to try to slow engine speed down at highway speeds, to get better mpg. What would be a good tranny swap for this?
Clark

A cheaper and easier way to go would be to change the gear ratio in the differential.
That engine has enough power that a 3.50 or 3.43 ratio should not kill it if you are doing any towing.
One problem may be finding gears.
Another way to go is find a 5 speed trans that will fit

Thanks, I do not tow much with it. The truck is basically a homeowners yardwork truck. The most I tow with it is a car on a flatbed car trailer. Put dirt or gravel in it sometimes. With it being diesel I may start driving it back and forth to work, if I can get it to better than 20 to 25mpg, it runs about 17mpg now with the current gearing.
Clark