those old B4 Passats were specifically designed in a wind tunnel to be as "slippery" as possible within reason back in 96. they have a very low drag coefficient for its time. but you must make sure ALL the plastic under body panels are in place. a good 2.25" exhaust with a muffler helps. tectonics tuning made a nice S.S. exhaust with magnaflow muffler, its good (although flex pipe is crap, ended up welding in a ball and socket joint for ford explorer, no more broken flex pipes). (i found i lost economy with straight pipe exhaust) the biggest bang for the $$ is a modded tune with start of injection advanced by a few degrees around 1400-2800rpm at cruising speed, a good tuner will take logs of how you drive and tailor them to meet your needs. also a good set of bosio nozzles (either stock or one size up) installed by a pro and pressure BALANCED!. good synthetic oil, changed every 5k miles or less. euro intake with no EGR from the European 110hp equivalent. dont remeber what mine was off of, got it over 20 years ago. (although some say EGR in tact and functioning can improve MPG.) take the "snow screen" out of air filter box. advance pump timing with VCDS all the way up to green line or ever so slightly above. make sure cam to crank timing is good, while lock is in end of cam and cam pulley is still loose. add feeler gauges under the back side of cam lock tool closest to firewall until all slack is taken up. then tighten cam sprocket and recheck timing, this will ever so slightly advance cam timing and increase MPG. install LED lights throughout car. this will alleviate the alternator from working so hard. 185/65/14 tires inflated to 38psi in rear and 40psi in front. a GOOD alignment at a reputable shop, not (wallmart or goodyear) with camber and caster and toe adjusted ON ALL 4 wheels. a good shop should know you can loosen subframe bolts and adjust subframe for caster adjustment in front, but a quallity alignment is going to be about 3-4 hours labor. a 92deg'C thermostat is a must for better economy. 5th gear upgrade, too bad the .681 is not available anymore, but anything is better than stock. and drive easy with smooth soft starts from stop, no full throttle, shift below 3000rpm. keep it below 70-75MPH im sure i missed some things, but with these mods i routinely achieved low to mid 60's MPG in summer i could get 61-2ish with A/C on. in spring and fall with 40-60Deg'F outside air i could get as high as 67MPG. in winter it suffered, i could only get at best 52-54MPG but more like 50 most of the time, they really water the diesel down in N.Y, for the winter. hope this helps
