I am just curious why the Mustang has Pirelli's for 17" wheels, and BFGoodrich for 18" wheels. On a side not (sorry I am in a tire state of mind at the moment) what do the numbers (such as 300A A a vs. 400AA A) next to UTQG represent, treadwear?

 

Price and rating.

 

The 300 and 400 are treadwear ratings. 300 will theoretically last 3/4 as long as 400. I'm seeing that my Pirellis rated at 400 look like they will last for 40K miles, since there is are 30K miles on my car now (and that includes one really hard day at an HPDE event at Lime Rock race track ). So, if these Pirellis last 40K miles, I'd expect the 300-rated BFG tires to get roughly 30K.

 

Cant compare tread wear ratings with different manufacturer's

 

This seems to be correct! Thanks for the info.
Treadwear Grade Number
The treadwear grade describes how long the tire manufacturer expects the tire to last. A Course Monitoring Tire (the standard tire that a test tire will be compared to) has a rating of "100". If a manufacturer assigns a treadwear rating of 200 to a new tire, they are indicating that they expect the new tire to have a useful lifespan that is 200% of the life of a Course Monitoring Tire.

[edit] Limitations

The DOT does not test tires. It depends on manufacturers to test their own tires and report the results. Unfortunately, this system has made treadwear ratings far less useful than the DOT had originally intended because tire manufacturers are able to use the treadwear grade as a marketing tool.
It is legal and permissible for a manufacturer to give their tire a 240 rating when their competitor's equivalent tire has a 220 rating; thus creating the false impression that the 240 tire is a better purchase because it will last longer. This tendency to inflate treadwear numbers has become so common that some manufacturers may report that ALL their tires have above average treadwear grades. Some are taking normal tires and reporting a treadwear of 600 or more, or giving a 220 rating to maximum performance tires with a reputation for poor tire life (e.g. the Goodyear Eagle F1 Supercar EMT).

 

There is definently a difference between brands. I believe nitto 555r's and mt et streets have the same wear # (100) but the nittos last WAY longer than the mt et streets.