Under no definition, by any governing body or dictionary, does 'being faster' have anything to do with 'doing it more often'.
World Recrods for being faster, higher or stronger don't care about 'how often'
WHen kids argue about which car is faster, no one cares how often they actually get driven. The Bugatti Veyron supersport is the fastest production car in the world, the reality is there are only a handful of them in existence and each one probably only ever gets driven a few times a year, and almost never at anything approaching their top speed. To be honest, other than VW's test drivers, and the one time on Top Gear, it's probably never been driven at top speed. But it doesn't matter, because it only had to do it once to break the record.
Thrust SSC is the fastest land vehicle in existence (I refuse to call it a 'car'). It only ever took one attempt at the record, broke it, and has has been sitting in a museum ever since. Until someone else comes along that can break the record, it still The Fastest.
No reasonable person is ever going to accept any kind of definition that tries to redefine faster to mean 'more often'. Nascar could have two races every week of the year, for 104 races. If the fastest they reach all year is 192 mph, that's it. The 103 times the go somewhere between 90 and 180 are irrelevant for the point of discussing 'fastest'. If another series races on once all year, and goes 193 mph, then they are faster and that is the end of that.
Now if NASCAR's were faster than Indycar's on some kind of tracks, then that would be relevant. But Indycar's fastest speed will be SIGNIFICANTLY faster on all three types of tracks that they both race on (super speedway, oval and road), and they would be faster on street's as well. In fact, I wouldn't be surprised if the Indycar's end up being faster on the faster street courses than NASCAR are at Sears Point... and possibly even Martinsville, if you want to use qualifying lap speeds as the reference mark. (considering pole last year for St Pete. was 104.5mph, it almost certainly will be)
The reality is, even without the above, it all boils down to one cold hard fact. Who's fastest official lap (race or qualifying) turned all year will be faster... NASCAR's or Indycar's? barring an extremely unlucky turn of weather for Indycar, I think the result is obvious.