Horrid gas mileage with a 91' Nissan D21, 4x4, 5-speed.

EVAP failure.
this means the gas tank venting system is leaking or the tests parts
bad. the FSM covers all tests. to find true cause. and is always here

http://www.scantool.net/software/alldata-diy-1-month-subscription.html

You need to either replace the mileage or replace the cluster

I believe it is something they are putting in the gas. I have 2 cars that are getting worse mileage now than in the summer. 1 is a Prius the other is a Pontiac Vibe. The Prius used to go 450 miles on a full tank now i'm lucky if i get 300. I also asked a co worker and he said the same thing, less mileage. I wouldn't get nuts trying to find something mechanically wrong with your car.

completely unrelated
run the fault codes first then consider a problem heated oxygen sensor/s
consider a blocked cat converter/s
your stated average consumption is 20 mpg so you are either speeding or have a problem to start with
That car should be around the 400 miles mark per tank

If the fluid level is going down then you do have a leak.

Gas milage (economy) is dependent upon the O2 Snsr's (oxygen Sensors) input to the ECU (Engine Control Unit). The cam position sensor, if malfunctioning, it should be replaced (cleaning it doesn't repair the problem which exists inside the component/sensor). Before you start throwing money after parts, Have a qualified/certified Hyundai Service Technician properly diagnose this issue.

Possible timing issue, I don't think the car uses a choke. Poss vacuum leak. start by having the t belt rechecked by the installer.Also ck the trans fluid

CURIOUS IF YOU ARE REFFERING TO THE ECM UPDATE THEY OFFERED FOR FREE ? IT WILL REALLY MAKE YOU WONDER? CHECK YOUR MASS AIRFLOW SENSOR- IF YOU GET A DTC THAT STATES YOUR TOO LEAN TO SYS BANK 1 REPLACE- BE CAREFUL THOUGH THE MAF SENSOR CAN BE EXPENSIVE-ORILEYS SEEM TO BE THE CHEAPEST AND DO NOT BUY THE SENSOR WITH THE HOUSING. 419.00 VS. 180.00 ITS ONE CLIP AND I BELIEVE 2 SCREWS. VERY TRICKY CAR SO BE CAREFULL . ITS A VERY DETAILED PROCESS THAT HAS TO BE FOLLOWED SO IF YA ARENT WILLING TO RISK IT TAKE TO
A DIFFERENT NISSAN DEALERSHIP ASK FOR AN ESTIMATE FOR REPAIRS AND A PRINTOUT OF ALL REPAIRS MADE BY NISSAN- FREE AND VERY GOOD TO HAVE- TRY THESE AS I HAVE AND SEE IF YOU GET ANY RESULTS;
CHECK BELT TENSIONER...
CLEAN FUEL INJECTION SYSTEM.
UPDATE YOUR ECM TO ACCOMODATE NEW INTAKE AND EXHAUST
REPLACE MAF SENSOR-
CHECK EHAUST FLANGE- MOTOR IS KNOWN FOR ITS ABILITY TO PRODUCE EXHAUST MANIFOLD LEAKS- WITH HOOD UP ITS THAT PUTTER SOUND NOT VERY LOUD AT ALL BUT GAS MILAGE IS REDUCED
CLEAN GAS TANK
GET GAS AT ANOTHER STATION- I FOUND THAT IF I SPEND THE EXTRA DOLLAR OR SO PER TANK ON 91-93 OCTANE MY GAS MILAGE PER TANK INCREASES 200 MILES EASY
CAR MANUAL EXPLAINS EVERYTHING SO IF YOU HAVE OWNERS MANUAL READ THRU TROUBLESHOOTING BEFORE SPENDING LOTS OF TIME N MONEY YOU DONT NEED TO . YOU HAVE THE ANSWERS IN THE BOOK . ALLDATA.COM HAS A GREAT MANUAL AVAIL ONLINE ALSO YOU MAY CHECK LOCAL LIBRARY. DO SOME HOMEWORK SAVE SOME MONEY

Take your foot of the gas. Fast starts ie speeding away from stops and starts burns more fuel.
Make sure your air filter is clean to allow proper fuel air mix ratio. If dirty replace it.
Lighten your load.Don't carry around a lot of dead weight in your truck. The heavier the load the more gas used.
Use a 91 or higher octane gas, this gas will burn hotter and give you more power thus again needing to step on the throttle less to create momentum
Try to maintain speed limit. speeding cost more.

See if you have a leak in any of your coolant tubes. If no leaks, you may have a bad thermostat that is stuck closed and needs to be replaced asap.