When we were doing the T.M. Tire thread on here we tried to clear some of this up. First I would really like to thank both Tony of T.M. Tires and Frank C. of CRC for their huge help on this subject. I think Frank's site does a great job of dealing with a subject that most racers never had/have a clue on and Tony is extremely knowledgeable as well.
From what I understand (and please contact either T.M. Tires or CRC for more info, they're the experts, not me):
3 types of "families"
1 = Natural rubber. Red
2 = Mostly Natural rubber with a foam mix (synthetic?) Pink, Magenta(aka
bl. Pink), Purple
3 = Mostly Foam with some natural rubber mix. White, Gray, Black, Blue
1 = Needs heat/friction to develop traction
2 = Need heat/friction to develop traction but does gain some traction from additives
3 = Need traction additives to develop traction
2= Medium on carpet but higher on asphalt (this seems to be due to less additive loss as a result of a decreased wear rate and increased friction from the surface type being asphalt)
3= High on carpet but lower on asphalt over the course of the run (this seems to be attributed to increased tire wear plaining off too much of the treated foam and not getting most of its grip from heat/friction. Treated=additive soaked)
1= Very low wear
2= Medium wear (12th mod rear = 5-7 run average of 1.80" down to 1.75"/Carpet) note: less on asphalt
3= High wear (12th mod rear = 4 run average of 1.80" down to 1.75"/Carpet) note: less on asphalt
Tony of T.M. Tires says that tire durometer readings can vary upwards of 5 points and that readings should be taken in the center of the tire at a constant tire diameter of 1.90". Some manufacturers may take their shore readings at different tire diameters.
Tony also notes (if I remember correctly) that as the tire gets smaller from this diameter the shore reading increases. Tony said that readings done on the outside of the tire are softer due to rim flex and harder on the inside due to decrease rim flex, thus taking them in the middle.
Finally, if I remember correctly, Tony has stated that not all tire durometers are created equal and that it can be difficult sometimes for racers to get consistent results with the small hand held units. Hope this helps.