the 2.5NA, like all diesels have HUGE air flow, and the stock air filter flows plenty. get a manometer and drive the car with stock air filter, and the K&N see if there is ANY difference. the cleanable ability of the K is cool, if you are cheap and cannot afford the 4.00 every 15000 miles or so...
performance improvements in this car with electric fans are negligible. the alternator drain will exceed the power saving from the fan. now, if you could install electric WATER PUMP, and fans, and you were in a race car with no alternator, then power improvements would be measurable. but on the road, and with alternator drain, the difference is not.
synthetic oil, proper balanced injectors and clean fuel filters, balanced fuel pressure, manual transmission with overdrive, adjusted ADA if your car has it... that about tops the performance economy mods you can do.
air dams, body kits, hard skinny tires, aerodynamic hubcaps, floor smoothing... search for hypermiling tricks and tips...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!