What do the symbols next to car stats mean?

As far as I can tell, these symbols are never explained in the help menu. They seem to be indicators of quality (listed from best to worse, based on my experience):

• Concentric circles
• Circle
• Triangle
• X

Are these all of the possible symbols? Do they have precise meanings, or are they only vaguely representative of the attribute's quality?

You've already worked out the order of quality, these just seem to be a quick measure of how good the car is in a particular mode.

For On-Road, Off-Road and Icy Road you can get a better picture by - when starting a race - going to the change vehicle screen to select a car to race. At the bottom of this page, under the car parts, is a "Road Effect" score give as a positive or negative percentage (presumably a performance modifier of sorts)

My rough experience so far is:

• Concentric circles - positive bonus.
• Circle - no bonus.
• Triangle - slight negative bonus.
• X - large negative bonus.

Unfortunately this doesn't help a great deal for the Adv or Rep scores, which I cannot find a better measure for and we can only assume the same rough measures.