It's is something that literally jumped out at me while doing some spreadsheet work.
I took the standard Saffir-Simpson Category numbers. Placed them on a spreadsheet. And filled in the gaps at 5mb intervals.

(NHC/TPC No longer uses minimum central pressure to classify Tropical Cyclones. That practice ended in the mid 1990s)

Using the above table:
980mb= 95mph or 82 knots
965mb=110mph or 95 knots__(15mb drop = 15mph increase)
945mb=130mph or 113 knots_(20mb drop =20mph increase)
920mb=155mph or 135 knots_(25mb drop = 25 mph increase)

After a Storm reaches 75mph. There is roughly a 1mph increase in windspeed for every 1mb drop in pressure.
This is what I call the "Maximum Possible Windspeed" in relation to pressure and wind Only.
Both Katrina and Rita had Maximum Windspeeds OVER the Max Possible Windspeed.
In other words Katrina and Rita went above the Pressure : Wind curve.

For those that have a spreadsheet. Excel or other versions.
Row 1,First Column- 1013 (mb is column name)
Row 1, Second column-10 (mph is column name)
Row 2,First Column- 1012
Row 2,Second column- 15
Now use the "add" function to take Column 1 down to 875mb, and Column 2 to 200 mph.

You should now have the HurriScale table.

This is Not a proven formula. But you can compare it to The Supercanes of 2005. The basis for the formula is the Atkinson-Holliday formulas and theories of the early 1970's. And some other authors over the last 30 years.

I'm working on an easy spreadsheet that I can post.

http://www.google.com/search?hl=en&q=pressure+wind+relationship

http://ams.allenpress.com/archive/1520-0493/105/4/pdf/i1520-0493-105-4-421.pdf

http://www.aoml.noaa.gov/hrd/tcfaq/D1.html

http://www.unc.edu/~rowlett/units/scales/saffir.html