flog

From the Wikipedia article on gear ratios we learn that:
v_car = (C_tire * v_engine) / (gr_gear * gr_diff)

For those of us with no tach, RPM can be inferred using the following formula:
v_engine = v_car * (gr_gear * gr_diff) / C_tire

In cars with transaxles, gr_diff is the final drive ratio, otherwise it'd be the gear ratio in the rear differential.