engine swap

does anyone know everything i would need to put a 6-cylinder in my 1991 s-10 pickup, it has a 4-cylinder and seems to be very underpowering, and i think the six cylinder will give it better gas mileage, since the 4-banger cant pull the truck up to speed very well.

You'll need the motor, motormounts, transmission, underhood wiring harness, complete exhaust, and possbly the complete fuel system.

Getting a complete good running truck will make this job more affordable

About a month of some serious spare time.

It's the exact same amount of work to drop in a V8, and even around the same amount of money.

Oh, your fuel economy will probably go way down, although some v6 S-10's do get decent mileage. I think that keeping the V6 in perfect tune is way more important than the 4 banger for good mileage.

if you want mileage keep the 4 cylinder

will the 4 banger actually give me better gas mileage than the 6-cylinder i want to put in it.

Yes' as long as they're both tuned up the 4 banger should always give you better mileage.

If you want the best fuel mileage, and can somehow find one Chevy had an optional 2.2L Diesel motor in the S-10 for a while.
I don't know what years these were available, and I only know of 1 person that actually has one. I don't think that he'd ever sell it

In the older models 82-93 the mileage were factory rated at the following
2.5 4cyl-5 speed- 23city/27hwy @ 95hp/130ftlbs
2.5 4cly-auto - 20city/26hwy @ 105hp/135ftlbs
2.8 6cly-5speed- 19city/25hwy @ 125hp/150ftlbs
4.3 6cly-5speed- 17city/22hwy @160hp/230ftlbs
4.3 6cyl-auto 17city/22hwy @195hp/260ftlbs