Relentless Inquiry

Some folks were discussing bad-weather road techniques and info on the Wanderlodge list, and this interesting snippet emerged:

Allegedly, the hydroplane speed of a vehicle in a 'hard rain' can be calculated as the square root of the tire pressure, multiplied by 9. So the Wanderlodges, which use 10.5/R22's in G or H load rating, at 100 psi, are just not going to hydroplane unless you can somehow get them up to about 90 mph. Which some of the folks on the list come close to doing– more than one of the DD 8v92 folks admit to cruising at 75 “or more” when doing a long haul move.

However the standard passenger car, at a customary PSI of around 36, has an excellent chance of hydroplaning at 55 mph, contrary to what many folks say (just slow down to 55 or 60 and you'll be fine). *Very* interesting!

Anyone else heard anything like this formula, or have a pointer to info that debunks it? Armchair physics of the form “I just know intuitively it couldn't, because xyz” is not welcome– looking for substantiated stuff.