Got my '97 in early may from NickS (
megaladon6) with 174k on the clock. Only performance mod is balanced pp520 nozzles and IQ adjustment when installed unless you count dropping the exhaust behind the mid-pipe. He said he was getting 43-45mpg (IIRC) mixed driving with syn-oil & power service every other tank. He got it at 105k and never cleaned the IM (I'm at 184k and haven't cleaned it either).
My daily round trip is 110 miles.
I use cruise whenever I can.
I don't use AC - windows cracked; vents open wide.
The belly pan is missing.
Early on I got 46-48mpg per tank (85-90% highway 65-78mph) mixed traffic (Newtown <--> Hartford) with or without PS on 215/40/17 tires.
Changes made (in sequence):
Add 4oz PS every fill - 48-49.5mpg
Stock steelies & POS snows 195/60/14 @40psi - no change
lower shift-point RPM (1.5kdown/2kup) & highway MPH by 5-12 (not above 68mph) - 53.4-53.8mph last 2 tanks & looking about the same on the 3rd.
Changes I'd like to make to improve further:
Kumho Ecsta LX Platinums (touch better than the Solus KR21 IMHO)
2.5" hi flow dp/cat/res/pipe exhaust
.681 5th (1st-2nd syncro grind + original clutch) when/if trans/clutch work needed.
bigger CAI (& possible IC/piping)
underbody flow improvement if I move the drivetrain out of this "road warrior" body (mk2 Jetta preferred)
I drove my stock '98 150k without cleaning the IM (it's ugly in there), adding PS with a 70m daily round trip and alternate weekend 450m jaunts to Cape Cod. Started at 48-50mpg and ended at 40-44mpg.
My uneducated experience tells me that nozzles & IQ adjustment picked up 2-4mpg, PS helps with the IM as well as picking up 2mpg and lowering speed & shift points can pick up 4-6mph.