Lv1 to 2 for 1 star tanks needs 100 exactly to level. You can see the needed xp to level in the upgrade menu.
A Lv13 one gave 2218 xp, and so did every other 1star tank at Lv13. There does not seem to be any benefits using same type tanks (usually there is in these games).
It definitely doesn't seem cost effective (more exp spent for less) but it does seem like the quickest way to level so far. Will probably have to make a spreadsheet later if I'm not feelin lazy.