From what I have read on LG, I have known that North American 4 foot F34T12 fluorescent tubes and 3 foot F25T12 fluorescent tubes save energy by using a krypton gas fill to lower their arc voltage drops just like European F18T8, F36T8, F58T8, and F70T8 fluorescent tubes use a krypton gas fill to save energy by changing the power factor so that energy saving can be achieved without reducing the arc voltage drop of the tube to prevent ballast failures.

I would like to know if there are any differences in the krypton gas fills between the North American energy saver rapid start T12 fluorescent tubes vs the European energy saver preheat T8 fluorescent tubes?

The main difference is the voltage required for ignition: The T8 won't start on a RS ballast, the OCV is way too low (need 600Vpeak even on a HF ballast, even more on a smooth 50Hz; the starter kickback is more close to the HF in the edge steepness here). In fact the fill must be different between the F34T12 vs F36T8, as they have very minimal difference in arc voltage, but way different shape (about 1:2 factor of their tube cross section) and the shape does affect the voltages (all, ignition, as well as running) as one of the major contributors. Dunno if that is just the pressure, or the overall composition.

As far as I know the fill composition is same for both, at 75:25 Kr:Ar.  Indeed the difference in tube diameter has a major impact on ignition and running voltage. 

Isn't then the fill pressure different? Something has to compensate for the different tube diameter, 36W (~93V) vs 34W (~88V) sounds to me as too small difference (barely 6%; sounds to me like even less than the manufacturing tolerance of either type) for T8 vs T12 (factor of ~2 in tube cross section area)... Or has the tube diameter really so little impact on the running arc voltage (while so huge on the ignition one)?

Medved, as far as I have been aware, F36T8 tubes have an arc voltage drop of 103v while F34T12 tubes have an arc voltage drop is actually 84v. In that case, the arc voltage drop difference seems more significant than what you were thinking because F34T12 fluorescent tubes have been known to overheat older North American F40T12 preheat and rapid start fluorescent tube ballasts since F40T12 fluorescent tubes have an arc voltage drop of roughly 100v and the lower arc voltage drop of the F34T12 fluorescent tube would cause the older ballasts to draw more current than they were designed to handle. On the other hand, F36T8 fluorescent tubes have never caused any North American F40T12 preheat fluorescent tube ballasts to overheat as well.

Here is a spec sheet that shows that the arc voltage drop of a F36T8 fluorescent tube is 103v:

https://www.lighting.philips.com/api/assets/v1/file/PhilipsLighting/content/fp927982284036-pss-global/927982284036_EU.en_AA.PROF.FP.pdf

Here is the bulletin that shows that F34T12 fluorescent tube has an arc voltage drop of 84v:

https://www.scribd.com/document/110155293/Westinghouse-Guide-to-Fluorescent-Lamps-Brochure-1979