That's a "6 Million Dollar Question"!
Many before you have asked this same question, however, either many of these have learned to live with it, or have been waaay too ignorant to post up the fix?
This link https://www.google.co.uk/search?q=Toyota+Supra+fault+code+89&rlz=1C1CHBF_en-GBGB703GB703&oq=Toyota+Supra+fault+code+89&aqs=chrome..69i57.5026j0j7&sourceid=chrome&ie=UTF-8
Will take you to links for several different possible error 89 fixes including;
Odo swap
Delimiter/speed-converter &/or it's wiring
Loom fault
TPS setting
Traction-Control stepper-motor operation
A speed-sensor
Auto to manual conversion result
Traction Control ECU
Engine or/& Transmission ECU.
Please post up which one worked for you?
Thank you.