Fiberglass isn't always lighter. If properly reinforced it's sometimes heavier. The Turbo MR2's fiberglass engine cover weighs almost twice that of the N/A's steel cover.
Weight loss on the MKIII is HELL. I think there's a thread in the FAQ section about it... if not search the MKIII forum.
You don't give other specifics about your car. Is it N/A or Turbo? If you're a very mellow driver you could swap in the LSD from a 1989 or later Turbo MKIII. It's geared 3.73:1 and will lower your RPM while cruising on the highway significantly. As for city driving well, your off the line performance will suffer and you'll have to step on it more to get up to speed.... Short of swapping a 1G or perhaps 5S or 3S (all 2 liter engines) I doubt you'll ever see much better mileage.
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