If I remember correctly, the difference was about $4000 dollars.
The GT came standard with options such as cruise control, power lumbar, power mirrors and power windows. The ground effects allowed for fog lights. It's been 12 years since I bought mine, so there may be more. The seats may be different. Jim can probably answer that. Did the LX come with an adjustable thigh support?
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351W 89 Mustang GT Convertible