140mph speedometer conversion?

I'm running 373 gears with a new 140mph speedometer, i've been told to use a 23 tooth gear. Is this correct?

check out www.50resto.com. They list speedo gears for many different gears.

Is the 140mph speedo a new calibrated speedo or just the faces are 140mph? Cause if it is just a 140mph guage face you need to get the old speedo recalibrated for 140mph. After that 140 or 85mph speedo does not matter, what matters is gearing 3.73 in your case and what tranny and year the tranny is.