I'm working with a Nissan Z24i I4 gas engined 4x4 that I hope to coax 25 mpg mixed from.

Baseline is around 20 mpg mixed. Have it up to almost 22 mpg mixed.

All I've done so far is to change out the gear oil in everything and add a moly additive. Moly in new engine oil and new plugs/cap/rotor. Fixed a sloppy timing chain. Upped the tire pressures.

For your diesel might try:

* Condensator or PCV jar

* acetone or Diesel Kleen with Cetane Boost

* moly additives in gear oil and engine

* spiral muffler

https://www.spiralturbobaffles.com/semiinfo.html

* fuel heater and 195F thermostat

* airtabs

best thing you can do in a diesel is turbo and supercharge the hell out of it. Diesel can run way leaner than gas, as you can already tell from 19 mpg in a one ton truck. Not to mention it has higher BTUs so better mileage even NA. You have a SG2 for that 2002? I agree with the clench, if you can help it don't drive a beast. Get something economical for DD or even all the short trips that aren't for work or whatever. It adds up.

I was standing along the side of the road out in the country when some guy in a "slammed" S-10 or Ranger 2wd pickup came by. He had it lowered and handling real fine...no screaming rpms...just whish thru the doglegs. Didn't even hear him coming.

I'd bet that with a low air dam on front and staying off the pedal it would do real well in the mpg dept. Wouldn't haul much though.

He also had an undercover cop on his tail....doubt if he caught up.

True, but a pickup will never run 100% loaded, even semis have some empty miles, so given how simple improvements can be, it's worth changing some stuff as long as it's easy to undo the changes and do truck stuff as well.