Ive never seen a reference to MPH in regards to timing in any of the eec files. By 90mph there could be other issues in reqards to load factors, injector pulse width, or ignition problems, curve etc... There are many things you can learn if the finances allow for the purchase.
I won't rule out the VSS signal (speed sensor on the speedometer cable at the trans) could be bad, or that in those early models there may be something that controls it, but it is highly unlikely. If you have no VSS sensor, or cruise control, then it is impossible anyway.
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