Why does your 93 civic Si steering wheel vibrate above 70 mph?

The vibration you're most likely experiencing is called "shimmy". It is a rotational vibration in the steering wheel due to an imbalance in your FRONT tire/wheel assembly (TWA - the combination of your tire and rim.) Centrifugal forces are created due to the imbalance, and these forces travel through the axle, the rack and pinion, then finally to the steering wheel.

Anyway, on to the important part: all you really have to do is re-balance your wheels. Whenever you change your tires (e.g. from all-seasons to winter and vice versa), the technician will (almost) always re-balance your wheels, so the problem should go away once the change of season comes. Alternatively, you can go ahead and just get a technician to re-balance your wheels at any time.

There's no need to rush to go and do it though. The consequences of not having properly-balanced TWA's is mainly the annoyances of shimmy at the 65-75 mph range. Above and below these speeds, the vibration should be fairly unnoticeable, and the wear due to shimmy is not significant enough to be much of a worry, as long as you eventually get it done.

Hope this helps!

Best Regards,

Tim Chak, Engineer

Honda R&D Americas, Inc.