Ok did some crunching and this is what I've found: XP is per mile on each given track, but based on roughly hitting an XP per minute rate for car class at an average racer's time. (This is all in Career Mode: I haven't checked Rivals, Free Play, or Multiplayer... but I think they're all the same.)
Road America, for example, is 605 XP per mile every time - which, for me, is 891 xp/min in C, 925/min in B, 980/min in A.
In general, my D class is 850-880xp/min (where I'm ranked around #40K); C class is 890-920XP/min (I'm around #100K); B class is 925xp/min (where I'm #150K) to 990XP/min (where I'm #11K); A class is 980-1010XP/min (I'm around #260K). Indy oval is my outlier: class X is 1500xp/min (I'm #8K).
XP per mile on the tracks I've checked: Indy 411; Atlanta 452; Road America 605; Spa 632; Prague 641; Sebring 649; Catalunya 727; Le Mans Bugatti 743; Laguna 759; Silverstone 768; Top Gear London 771; Long Beach Full 812.
CorruptedBomb you're right that, in general, a top-10% racer can average 1000xp/min in the faster classes and complete an affinity in 8 hours' career-mode drive time. But I think the average of all the classes, for a top-30% racer, is likely to be more like 800xp/min. This would be 600 minutes, or 10 hours' drive time per affinity.