_Prina, I find this quite interesting, and I've been lazily waiting for
a valid answer to your question--it's been on my mind, on a back burner I
must admit, but has nonetheless bothered me over the years.  The most
explanations I've received from tire dealers is the 'relativity
excuse'--i.e., a 200 theoretically goes 2 times the miles as a 100-rated
tire.  My dad sold tires and I sold cars.  At one time I would install the
el cheapo tires, like those with a 220 twr(treadwear rating), on cars I was
sending to auction.  After all, if they weren't Michelins, new tires were
just that--new tires (We only knew of 1 'quality' tire in my area.).  But
after I saw 2 customers wearing those same tires(not sold by me): (1) have a
blowout while sitting at a red light in hot weather; and (2) another have a
blowout riding in town at about 35 mph; both these tires blew out on the
sidewall, of all places.  I quit even using them on the auction-units!
   My customers and my family and I never have gotten the mileage wear
advertised by that belief in 30k per 100 rating.  One actually did get 60k
on a set of 60-k Michelins on a '95 Riviera, but I refused to ride with him
for the last 10-15k miles.  Like you, I feel there must be some percentage
we can use to help determine the actual miles to "really" expect.  Even
twr's will differ among mfgs--I don't wish to beat them down, but "Energy
Men" stores will put a 500 twr on a tire that won't give but about 60-70%
the miles that a 500 Michelin will give.
   My guestimate therefore would have to be for a given mfg.  Michelins,
for instance, I'd guess like you: using 500 to indicate 5 times the 30k, or
150,000 miles, I'd estimate about 1/2 the theoretical 150k, or 75k at
best--in our state, the last 2/32 inch of tread legally must be replaced!
My friend who got 60k from his Mich's was using Summits, and the 60k he
quoted me was what the selling dealer had told him--but the twr was way
higher than 200--which would've been twice the 30k from the 100 twr.  IIRC,
his twr was about 400.  Using the relativity model, that would suggest 4
times 30k, or 120 k.  And 1/2 of this 120k would be the 60k he actually
got--some miles from quality and some miles from prayers!
   I used to buy a metric-design tread from Western Auto--made pretty tires
for Taurus's etc.--and IIRC their twr was about 320.  We ran a set on an
~'87 model Taurus.  With about 20k on the tires, we sold it to a local
company.  Seems like they asked me to get them a new set of tires when they
had put about another 10k on them, for a total of 30k miles.  This rates
about 96k based on the 100 giving 30k, and works out to be a factor of about
1/3 actual miles as based on the 96k theoretical miles.  These were cheap
tires, and may have had a much-inflated rating like the "Energy Men" ratings
I mentioned earlier.
   Then one exception I must toss in: a friend who bought a new Explorer
and drove it on many daily 200-mile round trips, got an unbelieveable 130k
on a set of Firestones.  He then traded it to the local Toyota store, which
cleaned it up and put it on their budget lot--with the same tires, just
polished to give them a glossy appearance.
   My final analysis from all this bs is that one would at least have to
use a different set of parameters, and percentages, as he moves from one
mfg. to another.  But we can be assured of one thing: that model of 30k per
100 rating can NOT be relied upon.  Even if we stick to one brand, the %'s
are going to vary as we go from one temp. rating to another and from one
traction rating to another.
   Now to introduce another problem that prevents our getting as many
miles...those damnable flat spots that occur and create such roar and
bumping, even with 2/3 the original tread depth still remaining.  Can we
legally claim 2x their original cost?...or 3x the orig. cost?...as a
charitable income tax deduction?    I cried every time I had to "donate" so
many over the years that I'd think it reasonable to claim some % of their
original cost to pain and suffering!!!
   This whole theme seems to just defy all scientific criteria.  Unless we
defer it to "social science"!!!  All my 2 cents' worth, and often worth
every dime.  sam